This lesson is in the early stages of development (Alpha version)

Creating your own container images

Overview

Teaching: 20 min
Exercises: 15 min
Questions
  • How can I make my own Docker images?

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

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

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

  • Create an installation strategy for a container

  • 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 image.

Interactive installation

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

$ docker run -it alpine sh

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

/# python
sh: python: not found

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

/# apk add --update python py-pip python-dev

We can test our installation by running a Python command:

/# python --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 by default. There is a command that will “snapshot” our changes, but building containers this way is not very reproducible. Instead, we’re going to take what we’ve learned from this interactive installation and create our container 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 DEFUALT>

Let’s break this file down:

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 python py-pip python-dev
RUN pip install cython
CMD cat /proc/version && python --version

The recipe provided by this Dockerfile will use Alpine Linux as the base container, add Python and the Cython library, and set a default print command.

Create a new Docker image

So far, we just have a file. We want Docker to take this file, run the install commands inside, and then save the resulting container as a new container image. To do this we will use the docker build command.

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

$ docker build -t USERNAME/CONTAINERNAME .

The -t option names the container; 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 image alpine-python, I would use this command:

$ docker build -t alice/alpine-python .

Exercise: Review!

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

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

  3. What command will run the container you’ve created? What should happen by default if you run the 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 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 image.

  3. We want to use docker run to run the container.

$ docker run alice/alpine-python

should run the container and print out our default message, including the version of Linux and Python.

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

will run the container and print out “Hello world” instead.

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:

In general, a good strategy for installing software is:

TODO: Exercises

Have a set of “choose your own adventure” software installation examples

Share your new container on Docker Hub

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

$ docker push alice/alpine-python

Make sure to substitute the full name of your container!

In a web browser, open https://hub.docker.com, and on your user page you should now see your container 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 push doesn’t work for you, run docker login first, enter your Docker Hub username and password, and then try docker push again.

What’s in a name? (again)

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

You can rename images using the docker tag command. For example, imagine someone named Alice has been working on a workflow container 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 tag command would look like this:

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

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

Key Points

  • Dockerfiles specify what is within Docker images.

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

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