How to create docker image

How to Create a Docker Image From a Container

In this article, I’ll provide step-by-step instructions on how to create a Docker container, modify its internal state, and then save the container as an image.

This is really handy when you’re working out how an image should be constructed because you can just keep tweaking a running container until it works as you want it to. When you’re done, just save it as an image. You can use the following guide to customize and deploy the DataSet agent for common tasks such as adding parsers for your log files, adding redaction/sampling rules, running custom plugins, or mounting volumes for application log files.

Before we jump right into it, we’d like to invite you to a relevant webinar on how to get the most value out of your Kubernetes Audit logs. If you run Docker containers in K8s environments, we will cover the best practices to implement comprehensive, secure, and efficient audit logs in production Kubernetes clusters. We look forward to seeing in the webinar.

Step 1: Create a Base Container

Let’s get started by creating a running container. So that we don’t get bogged down in the details of any particular container, we can use nginx.

The Docker create command will create a new container for us from the command line:

Here we have requested a new container named nginx_base with port 80 exposed to localhost. We are using nginx:alpine as a base image for the container.

If you don’t have the nginx:alpine image in your local docker image repository, it will download automatically. When this happens, you will see something like this:

Step 2: Inspect Images

If you look at the list of images on your system, you will now see the nginx:alpine image:

Step 3: Inspect Containers

Step 4: Start the Container

Let’s start the container and see what happens.

Now visit http://localhost with your browser. You will see the default “Welcome to nginx!” page. We are now running an nginx container.

Step 5: Modify the Running Container

So if you wanted to modify this running container so that it behaves in a specific way, there are a variety of ways to do that.

In order to keep things as simple as possible, we are just going to copy a new index.html file onto the server. You could do practically anything you wanted here.

Let’s create a new index.html file and copy it onto the running container. Using an editor on your machine, create an index.html file in the same directory that you have been running Docker commands from.

Then paste the following HTML into it:

Then save the file and return to the command line. We will use the docker cp command to copy this file onto the running container.

Now reload your browser or revisit http://localhost. You will see the message “Hello World!” in place of the default nginx welcome page.

Step 6: Create an Image From a Container

So at this point, we’ve updated the contents of a running container and as long as we keep that container around, we don’t need to do anything.

However, we want to know how to save this container as an image so we can make other containers based on this one. The Docker commands to do this are quite simple.

To save a Docker container, we just need to use the docker commit command like this:

Now look at the docker images list:

You can see there is a new image there. It does not have a repository or tag, but it exists. This is an image created from the running container. Let’s tag it so it will be easier to find later.

Step 7: Tag the Image

Using docker tag, we can name the image we just created. We need the image ID for the command, so given that the image ID listed above is 0c17f0798823, our command will be:

And if we look at the index of images again, we can see that the s were replaced:
We can actually use complicated tags here with version numbers and all the other fixings of a tag command, but for our example, we’ll just create an image with a meaningful name.

Step 8: Create Images With Tags

You can also tag the image as it is created by adding another argument to the end of the command like this:

This command effectively commits and tags at the same time, which is helpful but not required.

Step 9: Delete the Original Container

Earlier we started a Docker container. We can see that it is still running using the docker ps command.

Let’s stop and remove the Docker container that is currently running and delete it.

If we list all of the Docker containers, we should have none:

Now, let’s create a new container based on the image we just created and start it.

Note that docker run is the equivalent of executing docker create followed by docker start; we are just saving a step here.

Step 10: Look at Running Containers

If you look at the running containers now, you will see we have one called hello_world:

As you can see, the index.html page now shows the “Hello World!” message just like we wanted.

Stop the container hello_world before moving on to the next section.

Step 11: Consider Your Options

There are a few optional things we can do using the commit command that will change information about our images.

For example, we might want to record who the author of our image is or capture a commit message telling us about the state of the image.

These are all controlled through optional parameters to the commit command.

Let’s go back to our original running container. We are going to use a slightly different command here to make cleanup easier:

This command will run the image nginx:alpine with the name nginx_base; the creation of the image will be included in the command execution.

So if you visit http://localhost now, you should see the default nginx welcome page.

We went through changing things about the running container above, so I won’t repeat that work here; instead, we want to look at the various options around the commit sub-command.

Option A: Set Authorship

Let’s start by setting the authorship of the image. If you inspect the docker image hello_world_nginx above, you will discover that its author field is blank.

We will use the docker inspect command to get the details of the image and grep out the author line.

So if we use the author option on the docker commit command, we can set the value of the author field.

And we can check the authorship of that image:

Let’s delete that image and try some other options:

Option B: Create Commit Messages

Let’s say you want a commit message to remind yourself what the image is about or what the state of the container was at the time the image was made.

There is a –message option you can use to include that information.

Execute this command:

Using the image name, we can look at the history of the Docker image to see our message. Here we are using the docker history command to show the change history of the image we created:

Notice that we see the entire history here, and the first entry is from our commit of the running container. The first line listed shows our commit message in the rightmost column.

Let’s remove this image and check out the other options:

Option C: Commit Without Pause

When you use the commit command, the container will be paused.

For our little play container this is unimportant, but you might be doing something like capturing an image of a production system where pausing isn’t an option.

You can add the –pause=false flag to the commit command, and the image will be created from the container without the pause.

If you don’t pause the container, you run the risk of corrupting your data.

For example, if the container is in the midst of a write operation, the data being written could be corrupted or come out incomplete. That is why, by default, the container gets paused before the image is created.

Let’s remove this image and check out the other options:

Option D: Change Configuration

You can change any of the following settings of the image during the commit process:

Nginx’s original docker file contains the following settings:

So we will just play with one of those for a moment. The NGINX_VERSION and EXPOSE could cause issues with container startup, so we will mess with the command line (CMD) executed by the container.

Now stop the nginx_base container with this command:

And start a new container from the image we just created:

The configuration for the nginx process will be dumped to standard out when you execute this command. You can scroll back several pages to find the command we executed.

Creating Docker Images: Conclusion

The docker commit subcommand is very useful for diagnostic activities and bootstrapping new images from existing containers.

As I showed above, there are many helpful options available, too. The Docker CLI has many other power commands. If you like, you can explore some of them here.

DataSet is the best-of-breed log analytics solution for dynamic container environments. It is the only solution that provides unmatched performance and scale while optimizing the total cost of ownership.

To find out more about working with Docker and DataSet, check out these resources:
Installing the DataSet Agent in Docker
Configure the DataSet Agent for Docker

How to Create Docker Image with Dockerfile

Home » DevOps and Development » How to Create Docker Image with Dockerfile

A Dockerfile is a script with instructions on how to build a Docker image. These instructions are, in fact, a group of commands executed automatically in the Docker environment to build a specific Docker image.

In this tutorial, learn how to create Docker image with a Dockerfile.

Install Docker

If you are interested in how to use a Dockerfile to create an image, you probably already have Docker installed on your system.

Note: Users who are just starting to use Docker can learn more about managing Docker containers, basic Docker commands, and how to update Docker image and container.

How to Create a Dockerfile

The first thing you need to do is to create a directory in which you can store all the Docker images you build.

1. As an example, we will create a directory named MyDockerImages with the command:

2. Move into that directory and create a new empty file (Dockerfile) in it by typing:

3. Open the file with a text editor of your choice. In this example, we opened the file using Nano:

4. Then, add the following content:

5. Save and exit the file.

6. You can check the content of the file by using the cat command:

Build a Docker Image with Dockerfile

The basic syntax used to build an image using a Dockerfile is:

To build a docker image, you would therefore use:

If you are already in the directory where the Dockerfile is located, put a . instead of the location:

By adding the -t flag, you can tag the new image with a name which will help you when dealing with multiple images:

Note: You may receive an error saying Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker…

This means the user does not have permission to access the Docker engine. Solve this problem by adding sudo before the command or run it as root.

Once the image is successfully built, you can verify whether it is on the list of local images with the command:

The output should show my_first_image available in the repository.

Create a New Container

Launch a new Docker container based on the image you created in the previous steps. We will name the container “test” and create it with the command:

The Hello World message should appear in the command line, as seen in the image above.

Using Dockerfile is a simpler and faster way of building Docker image. It automates the process by going through the script with all the commands for assembling an image.

When building a Docker image, you also want to make sure to keep Docker image size light. Avoiding large images speeds-up building and deploying containers. Therefore, it is crucial to reduce the image size to a minimum.

Now that you have learned how to create and use this document, study our extended list of Docker commands to explore additional possibilities Docker has to offer.

A Beginner’s Guide to Understanding and Building Docker Images

Last updated: Mar. 17, 2021

A Docker image is a read-only template that contains a set of instructions for creating a container that can run on the Docker platform. It provides a convenient way to package up applications and preconfigured server environments, which you can use for your own private use or share publicly with other Docker users. Docker images are also the starting point for anyone using Docker for the first time.

So, in this introduction, we’ll not only take you through the basics of Docker images, but also show you where to find ready-made, off-the-shelf images that will give you a head start in building your own containerized applications, tools, and services.

As a new Docker user, you’ll also need to understand how to build your own custom images. So, we’ll briefly cover how to create Docker images for deploying your code and assembling container-based services. But first let’s look at the composition of a Docker image in more detail.

THE BASICS: A Beginners’ Guide to Docker

Anatomy of a Docker Image

We’ll cover these two methods in more detail later in this guide. For now, though, let’s focus on the most important Docker image concepts.

Image Layers

Container Layer

Each time Docker launches a container from an image, it adds a thin writable layer, known as the container layer, which stores all changes to the container throughout its runtime. As this layer is the only difference between a live operational container and the source Docker image itself, any number of like-for-like containers can potentially share access to the same underlying image while maintaining their own individual state.

Containers based on the same image share that image, reducing resource overhead

Parent Image

Base Image

In simple terms, a base image is an empty first layer, which allows you to build your Docker images from scratch. Base images give you full control over the contents of images, but are generally intended for more advanced Docker users.

Docker Manifest

Together with a set of individual layer files, a Docker image also includes an additional file known as a manifest. This is essentially a description of the image in JSON format and comprises information such as image tags, a digital signature, and details on how to configure the container for different types of host platforms.

Scan your Docker images FOR FREE with JFrog CLI, in 3 easy steps

Scan Your Docker Images

Container Registries

Step by step guide: Set up a free private Docker registry

Container Repositories

Container repositories are the specific physical locations where your Docker images are actually stored, whereby each repository comprises a collection of related images with the same name. Each of the images within a repository is referenced individually by a different tag and represents a different version of fundamentally the same container deployment. For example, on Docker Hub, mysql is the name of the repository that contains different versions of the Docker image for the popular, open-source DBMS, MySQL.

How to Create a Docker Image

In this final section, we’ll cover the two different methods of creating Docker images in a little more detail, so you can start putting your knowledge into practice.

Interactive Method

Advantages: Quickest and simplest way to create Docker images. Ideal for testing, troubleshooting, determining dependencies, and validating processes.

Disadvantages: Difficult lifecycle management, requiring error-prone manual reconfiguration of live interactive processes. Easier to create unoptimized images with unnecessary layers.

The following is a set of simplified steps to creating an image interactively:

If you omit the tag name, then Docker automatically pulls the most recent image version, which is identified by the latest tag. If Docker cannot find the image locally then it will pull what it needs to build the container from the appropriate repository on Docker Hub.

In our example, we’ll launch a container environment based on the latest version of Ubuntu:

Next, you’ll need to know the name or ID of your running container instance.

The sample output below shows our running container with the ID e61e8081866d and the name keen_gauss :

CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES

e61e8081866d ubuntu “bash” 2 minutes ago Up 2 minutes keen_gauss

This name is randomly generated by the Docker daemon. But you can also identify your container with something more meaningful by assigning your own name using the – name operator in the Docker run command.

$ docker commit keen_gauss ubuntu_testbed

You should see your new image listed in the results.

REPOSITORY TAG IMAGE ID CREATED SIZE
ubuntu latest 775349758637 5 minutes ago 64.2MB

Dockerfile Method

Advantages: Clean, compact and repeatable recipe-based images. Easier lifecycle management and easier integration into continuous integration (CI) and continuous delivery (CD) processes. Clear self-documented record of steps taken to assemble the image.

Disadvantages: More difficult for beginners and more time consuming to create from scratch.

The Dockerfile approach is the method of choice for real-world, enterprise-grade container deployments. It’s a more systematic, flexible, and efficient way to build Docker images and the key to compact, reliable, and secure container environments.

In short, the Dockerfile method is a three-step process whereby you create the Dockerfile and add the commands you need to assemble the image.

The following table shows you those Dockerfile statements you’re most likely to use:

To specify the parent image.

To set the working directory for any commands that follow in the Dockerfile.

To install any applications and packages required for your container.

To copy over files or directories from a specific location.

As COPY, but also able to handle remote URLs and unpack compressed files.

To define which port through which to access your container application.

To add metadata to the image.

Example Dockerfile

An example of a Dockerfile for building an image based on official Ubuntu 18.04 with installing Nginx

The Docker Build Context

Finally, as we saw in the interactive method, you can use the Docker images command to see the image you’ve just created.

REPOSITORY TAG IMAGE ID CREATED SIZE

my-nginx 0.1 f95ae2e1344b 10 seconds ago 138MB

ubuntu 18.04 ccc6e87d482b 12 days ago 64.2MB

Again, you should see your new image listed in the results.

Orientation and setup

Estimated reading time: 5 minutes

Update to the Docker Desktop terms

Welcome! We are excited that you want to learn Docker.

This page contains step-by-step instructions on how to get started with Docker. In this tutorial, you’ll learn how to:

In addition, you’ll also learn about the best practices for building images, including instructions on how to scan your images for security vulnerabilities.

If you are looking for information on how to containerize an application using your favorite language, see Language-specific getting started guides.

We also recommend the video walkthrough from DockerCon 2020.

Download and install Docker

This tutorial assumes you have a current version of Docker installed on your machine. If you do not have Docker installed, choose your preferred operating system below to download Docker:

For Docker Desktop installation instructions, see:

Start the tutorial

If you’ve already run the command to get started with the tutorial, congratulations! If not, open a command prompt or bash window, and run the command:

You’ll notice a few flags being used. Here’s some more info on them:

You can combine single character flags to shorten the full command. As an example, the command above could be written as:

The Docker Dashboard

Before going too far, we want to highlight the Docker Dashboard, which gives you a quick view of the containers running on your machine. The Docker Dashboard is available for Mac, Windows, and Linux. It gives you quick access to container logs, lets you get a shell inside the container, and lets you easily manage container lifecycles (stop, remove, etc.).

To access the dashboard, follow the instructions in the Docker Desktop manual. If you open the dashboard now, you will see this tutorial running! The container name ( jolly_bouman below) is a randomly created name. So, you’ll most likely have a different name.

What is a container?

Now that you’ve run a container, what is a container? Simply put, a container is a sandboxed process on your machine that is isolated from all other processes on the host machine. That isolation leverages kernel namespaces and cgroups, features that have been in Linux for a long time. Docker has worked to make these capabilities approachable and easy to use. To summarize, a container:

Creating containers from scratch

If you’d like to see how containers are built from scratch, Liz Rice from Aqua Security has a fantastic talk in which she creates a container from scratch in Go. While the talk does not go into networking, using images for the filesystem, and other advanced topics, it gives a fantastic deep dive into how things are working.

What is a container image?

We’ll dive deeper into images later on, covering topics such as layering, best practices, and more.

CLI references

Refer to the following topics for further documentation on all CLI commands used in this article:

How to create custom docker images

Last updated: February 2nd 2022

Introduction

In order to run your application in a docker container, a customized docker image is created. This customized docker image includes instructions that install specific packages and copy the code into the docker container. This guide describes the basic steps to create a custom docker image.

Prerequisites

Writing Dockerfile for custom docker image

Docker builds the docker image by reading the instructions from a text file. By default Docker looks for a file named Dockerfile to build the docker image. The Dockerfile consists of instructions that are used to customize the docker image.

For this guide, we will write a Dockerfile for a node.js application. First go to the root directory of a node.js project.

Create a Dockerfile using the following command in the terminal.

Open the Dockerfile in your favorite editor.

Following is the format of Dockerfile instruction.

The instructions in Dockerfile are not case sensitive but it is convention to use UPPERCASE letters for instructions. Dockerfile builds the docker image by running the instructions in the order they are specified in Dockerfile.

A Dockerfile always starts from a FROM instruction which specifies which base image will be used to create the custom docker image. For example, if you want to create a custom docker image for node.js application, then the node base image will be used as follows.

If you do not specify a version tag, by default it will use the node image with the latest tag. The base docker image will be pulled from DockerHub if it is not available locally.

Dockerfile provides WORKDIR instruction to set the working directory.

The above instruction will set the working directory to /app inside the container. All the remaining instructions will be executed in this directory.

The COPY and ADD instructions are used to copy data into the docker container. The COPY instruction is only used to copy data from docker host to docker container while the ADD instruction can copy data from docker host and web as well.

Copy the source code into the docker container using the COPY instruction as follows.

This instruction will copy all the data from the working directory of the docker host to the working directory of the docker container.

In order to install new packages or run some shell commands in the base docker image, the RUN instruction is used. For example, in order to install npm packages, the RUN instruction will be used as follows.

The RUN instruction will run the command in the shell of the docker container.

The EXPOSE instruction is used to expose a port of a container. The port on which the application runs can be exposed using the EXPOSE instruction as follows.

Now the application running on port 3000 of the docker container will be accessible from docker host when a container is launched using this docker image.

The CMD and ENTRYPOINT instructions are used to execute the shell commands inside the docker container when the docker container starts.

While the CMD instruction is used to define the arguments passed to the shell command.

Following is the final Dockerfile.

Create docker image from Dockerfile

After writing Dockerfile, now run the following command in the terminal to build the docker image from Dockerfile.

This command will get the Dockerfile from the current working directory and build the docker image.

In order to build the docker image from another directory, specify the path of the directory containing Dockerfile.

This command will get the Dockerfile from /home/$USER directory and build the docker image.

To build a docker image from a file named other than Dockerfile, specify the Dockerfile name using the -f option.

The above command will build the docker image by reading instructions from Dockerfile.dev file in /home/$USER directory.

Each docker image created using the build command gets a unique ID and all the docker images with their IDs can be listed using the following command.

Tag docker image

The docker image built using the commands described in the previous section does not have a name and tag. Docker tags are helpful to push the docker image to a remote docker repository and specify the docker image version.

Use the following command to tag the docker image.

The above command will take a docker image with ID 21233 and add a tag node-app:v1 to it.

Use the following command to list all the docker images with tags.

Push docker image to DockerHub

In order to push the docker image to DockerHub or some other docker image repository, the docker image must be tagged properly. If you want to push a docker image to a docker hub repository, the docker image must be tagged as follows.

Where example is the docker hub account id, node-app is the docker hub repository and v1 is the docker image tag. Use the following command to tag the docker image.

Before pushing the docker image to docker hub, you must log into the docker hub using command line. Use the following command to log into docker hub.

It will ask for docker hub username and password. After authentication, use the following command to push the docker image to docker hub.

Conclusion

A customized docker image can be created by writing instructions in a file named Dockerfile. This guide explains how different instructions can be used in Dockerfile to create a custom docker image.

We have servers in: