Creating Your Own Container Images
OverviewTeaching: 20 min
Exercises: 15 minQuestions
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
Dockerfileand show some simple examples.
Demonstrate how to build a Docker container image from a
Compare the steps of creating a container image interactively versus a
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.
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
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?
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
If you haven’t already, exit out of the interactively running container.
Put installation instructions in 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
CMDin exec-form (see the
CMDsection 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
CMDinstruction 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/passwdis 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?
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
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
- 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:
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 .
-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
alpine-python, I would use this command:
$ docker image build -t alice/alpine-python .
Notice that the final input to
docker image buildisn’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 buildcommand. 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.
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?)
We didn’t specify a tag for our container image name. What tag did Docker automatically use?
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”?
To see your new image, run
docker image ls. You should see the name of your new container image under the “REPOSITORY” heading.
In the output of
docker image ls, you can see that Docker has automatically used the
latesttag for our new container image.
We want to use
docker container runto 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
apkcommand we used above is the software package installer for Alpine Linux. The installers for various common Linux distributions are listed below:
yumMost 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
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
Dockerfileand 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.
Technically, you have to be logged into Docker on your computer for this to work. Usually it happens by default, but if
docker image pushdoesn’t work for you, run
docker loginfirst, enter your Docker Hub username and password, and then try
docker image pushagain.
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
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
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,
docker image push alice/workflow-complete:v1
Dockerfiles specify what is within Docker container images.
docker image buildcommand is used to build a container image from a
You can share your Docker container images through the Docker Hub so that others can create Docker containers from your container images.