Creating Your Own Container Images

Overview

Teaching: 15 min
Exercises: 15 min

Questions

• How can I make my own Docker container images?

• How do I document the ‘recipe’ for a Docker container image?

Objectives

• Explain the purpose of a Dockerfile and show some simple examples.

• Demonstrate how to build a Docker container image from a Dockerfile.

• Compare the steps of creating a container image interactively versus a Dockerfile.

• Create an installation strategy for a container image.

• Demonstrate how to upload (‘push’) your container images to the Docker Hub.

• Describe the significance of the Docker Hub naming scheme.

There are lots of reasons why you might want to create your own Docker container image.

• You can’t find a container image with all the tools you need on Docker Hub.
• You want to have a container image to “archive” all the specific software versions you ran for a project.
• You want to share your workflow with someone else.

Interactive installation

Before creating a reproducible installation, let’s experiment with installing software inside a container. Start a container from the alpine container image we used before, interactively:

$ docker container run -it alpine sh

Because this is a basic container, there’s a lot of things not installed – for example, python3.

/# python3

sh: python3: not found

Inside the container, we can run commands to install Python 3. The Alpine version of Linux has a installation tool called apk that we can use to install Python 3.

/# apk add --update python3 py3-pip python3-dev

We can test our installation by running a Python command:

/# python3 --version

Once Python is installed, we can add Python packages using the pip package installer:

/# pip install cython

Exercise: Searching for Help

Can you find instructions for installing R on Alpine Linux? Do they work?

Solution

A quick search should hopefully show that the way to install R on Alpine Linux is:

/# apk add R

Once we exit, these changes are not saved to a new container image by default. There is a command that will “snapshot” our changes, but building container images this way is not easily reproducible. Instead, we’re going to take what we’ve learned from this interactive installation and create our container image from a reproducible recipe, known as a Dockerfile.

If you haven’t already, exit out of the interactively running container.

/# exit

Put installation instructions in a Dockerfile

A Dockerfile is a plain text file with keywords and commands that can be used to create a new container image.

From your shell, go to the folder you downloaded at the start of the lesson and print out the Dockerfile inside:

$ cd ~/Desktop/docker-intro/basic
$ cat Dockerfile

FROM <EXISTING IMAGE>
RUN <INSTALL CMDS FROM SHELL>
RUN <INSTALL CMDS FROM SHELL>
CMD <CMD TO RUN BY DEFAULT>

Let’s break this file down:

• The first line, FROM, indicates which container image we’re starting with. It is the “base” container image we are going to start from.
• The next two lines RUN, will indicate installation commands we want to run. These are the same commands that we used interactively above.
• The last line, CMD, indicates the default command we want a container based on this container image to run, if no other command is provided. It is recommended to provide CMD in exec-form (see the CMD section of the Dockerfile documentation for more details). It is written as a list which contains the executable to run as its first element, optionally followed by any arguments as subsequent elements. The list is enclosed in square brackets ([]) and its elements are double-quoted (") strings which are separated by commas. For example, CMD ["ls", "-lF", "--color", "/etc"] would translate to ls -lF --color /etc.

shell-form and exec-form for CMD

Another way to specify the parameter for the CMD instruction is the shell-form. Here you type the command as you would call it from the command line. Docker then silently runs this command in the image’s standard shell. CMD cat /etc/passwd is equivalent to CMD ["/bin/sh", "-c", "cat /etc/passwd"]. We recommend to prefer the more explicit exec-form because we will be able to create more flexible container image command options and make sure complex commands are unambiguous in this format.

Exercise: Take a Guess

Do you have any ideas about what we should use to fill in the sample Dockerfile to replicate the installation we did above?

Solution:

Based on our experience above, edit the Dockerfile (in your text editor of choice) to look like this:

FROM alpine
RUN apk add --update python3 py3-pip python3-dev
RUN pip install cython
CMD ["python3", "--version"]

The recipe provided by the Dockerfile shown in the solution to the preceding exercise will use Alpine Linux as the base container image, add Python 3 and the Cython library, and set a default command to request Python 3 to report its version information.

Create a new Docker image

So far, we only have a text file named Dockerfile – we do not yet have a container image. We want Docker to take this Dockerfile, run the installation commands contained within it, and then save the resulting container as a new container image. To do this we will use the docker image build command.

We have to provide docker image build with two pieces of information:

• the location of the Dockerfile
• the name of the new container image. Remember the naming scheme from before? You should name your new image with your Docker Hub username and a name for the container image, like this: USERNAME/CONTAINER_IMAGE_NAME.

All together, the build command that you should run on your computer, will have a similar structure to this:

$ docker image build -t USERNAME/CONTAINER_IMAGE_NAME .

The -t option names the container image; the final dot indicates that the Dockerfile is in our current directory.

For example, if my user name was alice and I wanted to call my container image alpine-python, I would use this command:

$ docker image build -t alice/alpine-python .

Build Context

Notice that the final input to docker image build isn’t the Dockerfile – it’s a directory! In the command above, we’ve used the current working directory (.) of the shell as the final input to the docker image build command. This option provides what is called the build context to Docker – if there are files being copied into the built container image more details in the next episode they’re assumed to be in this location. Docker expects to see a Dockerfile in the build context also (unless you tell it to look elsewhere).

Even if it won’t need all of the files in the build context directory, Docker does “load” them before starting to build, which means that it’s a good idea to have only what you need for the container image in a build context directory, as we’ve done in this example.

Exercise: Review!

1. Think back to earlier. What command can you run to check if your container image was created successfully? (Hint: what command shows the container images on your computer?)

2. We didn’t specify a tag for our container image name. What tag did Docker automatically use?

3. What command will run a container based on the container image you’ve created? What should happen by default if you run such a container? Can you make it do something different, like print “hello world”?

Solution

1. To see your new image, run docker image ls. You should see the name of your new container image under the “REPOSITORY” heading.

2. In the output of docker image ls, you can see that Docker has automatically used the latest tag for our new container image.

3. We want to use docker container run to run a container based on a container image.

The following command should run a container and print out our default message, the version of Python:

$ docker container run alice/alpine-python

To run a container based on our container image and print out “Hello world” instead:

$ docker container run alice/alpine-python echo "Hello World"

While it may not look like you have achieved much, you have already effected the combination of a lightweight Linux operating system with your specification to run a given command that can operate reliably on macOS, Microsoft Windows, Linux and on the cloud!

Boring but important notes about installation

There are a lot of choices when it comes to installing software – sometimes too many! Here are some things to consider when creating your own container image:

• Start smart, or, don’t install everything from scratch! If you’re using Python as your main tool, start with a Python container image. Same with R. We’ve used Alpine Linux as an example in this lesson, but it’s generally not a good container image to start with for initial development and experimentation because it is a less common distribution of Linux; using Ubuntu, Debian and CentOS are all good options for scientific software installations. The program you’re using might recommend a particular distribution of Linux, and if so, it may be useful to start with a container image for that distribution.
• How big? How much software do you really need to install? When you have a choice, lean towards using smaller starting container images and installing only what’s needed for your software, as a bigger container image means longer download times to use.
• Know (or Google) your Linux. Different distributions of Linux often have distinct sets of tools for installing software. The apk command we used above is the software package installer for Alpine Linux. The installers for various common Linux distributions are listed below:
  • Ubuntu: apt or apt-get
  • Debian: deb
  • CentOS: yum Most common software installations are available to be installed via these tools. A web search for “install X on Y Linux” is usually a good start for common software installation tasks; if something isn’t available via the Linux distribution’s installation tools, try the options below.
• Use what you know. You’ve probably used commands like pip or install.packages() before on your own computer – these will also work to install things in container images (if the basic scripting language is installed).
• README. Many scientific software tools have a README or installation instructions that lay out how to install software. You want to look for instructions for Linux. If the install instructions include options like those suggested above, try those first.

In general, a good strategy for installing software is:

• Make a list of what you want to install.
• Look for pre-existing container images.
• Read through instructions for software you’ll need to install.
• Try installing everything interactively in your base container – take notes!
• From your interactive installation, create a Dockerfile and then try to build the container image from that.

Share your new container image on Docker Hub

Container images that you release publicly can be stored on the Docker Hub for free. If you name your container image as described above, with your Docker Hub username, all you need to do is run the opposite of docker image pull – docker image push.

$ docker image push alice/alpine-python

Make sure to substitute the full name of your container image!

In a web browser, open https://hub.docker.com, and on your user page you should now see your container image listed, for anyone to use or build on.

Logging In

Technically, you have to be logged into Docker on your computer for this to work. Usually it happens by default, but if docker image push doesn’t work for you, run docker login first, enter your Docker Hub username and password, and then try docker image push again.

What’s in a name? (again)

You don’t have to name your containers images using the USERNAME/CONTAINER_IMAGE_NAME:TAG naming scheme. On your own computer, you can call container images whatever you want, and refer to them by the names you choose. It’s only when you want to share a container image that it needs the correct naming format.

You can rename container images using the docker image tag command. For example, imagine someone named Alice has been working on a workflow container image and called it workflow-test on her own computer. She now wants to share it in her alice Docker Hub account with the name workflow-complete and a tag of v1. Her docker image tag command would look like this:

$ docker image tag workflow-test alice/workflow-complete:v1

She could then push the re-named container image to Docker Hub, using docker image push alice/workflow-complete:v1

Key Points

• Dockerfiles specify what is within Docker container images.

• The docker image build command is used to build a container image from a Dockerfile.

• You can share your Docker container images through the Docker Hub so that others can create Docker containers from your container images.