Creating More Complex Container Images

Overview

Teaching: 30 min
Exercises: 30 min

Questions

• How can I make more complex container images?

Objectives

• Explain how you can include files within Docker images when you build them.

• Explain how you can access files on the Docker host from your Docker containers.

In order to create and use your own containers, you may need more information than our previous example. You may want to use files from outside the container, copy those files into the container, and just generally learn a little bit about software installation. This episode will cover these. Note that the examples will get gradually more and more complex - most day-to-day use of containers can be accomplished using the first 1-2 sections on this page.

Using scripts and files from outside the container

In your shell, change to the sum folder in the docker-intro folder and look at the files inside.

$ cd ~/Desktop/docker-intro/sum
$ ls

This folder has both a Dockerfile and a python script called sum.py. Let’s say we wanted to try running the script using our recently created alpine-python container.

Running containers

What command would we use to run python from the alpine-python container?

If we try running the container and Python script, what happens?

$ docker run alice/alpine-python python3 sum.py

python3: can't open file 'sum.py': [Errno 2] No such file or directory

No such file or directory

What does the error message mean? Why might the Python inside the container not be able to find or open our script?

The problem here is that the container and its file system is separate from our host computer’s file system. When the container runs, it can’t see anything outside itself, including any of the files on our computer. In order to use Python (inside the container) and our script (outside the container, on our computer), we need to create a link between the directory on our computer and the container.

This link is called a “mount” and is what happens automatically when a USB drive or other external hard drive gets connected to a computer - you can see the contents appear as if they were on your computer.

We can create a mount between our computer and the running container by using an additional option to docker run. We’ll also use the variable $PWD which will substitute in our current working directory. The option will look like this

-v $PWD:/temp

What this means is – link my current directory with the container, and inside the container, name the directory /temp

Let’s try running the command now:

$ docker run -v $PWD:/temp alice/alpine-python python3 sum.py

But we get the same error!

python3: can't open file 'sum.py': [Errno 2] No such file or directory

This final piece is a bit tricky – we really have to remember to put ourselves inside the container. Where is the sum.py file? It’s in the directory that’s been mapped to /temp – so we need to include that in the path to the script. This command should give us what we need:

$ docker run -v $PWD:/temp alice/alpine-python python3 /temp/sum.py

Note that if we create any files in the /temp directory while the container is running, these files will appear on our host filesystem in the original directory and will stay there even when the container stops.

Exercise: Explore the script

What happens if you use the docker run command above and put numbers after the script name?

Solution

This script comes from the Python Wiki and is set to add all numbers that are passed to it as arguments.

Exercise: Checking the options

Our Docker command has gotten much longer! Can you go through each piece of the Docker command above the explain what it does? How would you characterize the key components of a Docker command?

Solution

Here’s a breakdown of each piece of the command above

• docker run: use Docker to run a container
• -v $PWD:/temp: connect my current working directory ($PWD) as a folder inside the container called /temp
• alice/alpine-python: name of the container to run
• python3 /temp/sum.py: what commands to run in the container

More generally, every Docker command will have the form: docker [action] [docker options] [docker image] [command to run inside]

Exercise: Interactive jobs

Try using the directory mount option but run the container interactively. Can you find the folder that’s connected to your computer? What’s inside?

Solution

The docker command to run the container interactively is:

$ docker run -v $PWD:/temp -it alice/alpine-python sh

Once inside, you should be able to navigate to the /temp folder and see that’s contents are the same as the files on your computer:

/# cd /temp
/# ls

Mounting a folder can be very useful when you want to run the software inside your container on many different input files. In other situations, you may want to save or archive an authoritative version of your data by adding it to the container permanently. That’s what we will cover next.

Including personal scripts and data in a container

Our next project will be to add our own files to a container - something you might want to do if you’re sharing a finished analysis or just want to have an archived copy of your entire analysis including the data. Let’s assume that we’ve finished with our sum.py script and want to add it to the container itself.

In your shell, you should still be in the sum folder in the docker-intro folder.

$ pwd

$ /Users/yourname/Desktop/docker-intro/sum

We will modify our Dockerfile again to build an image based on Alpine Linux with Python 3 installed (just as we did perviously). This time we will add an additional line before the CMD line:

COPY sum.py /home

This line will cause Docker to copy the file from your computer into the container’s file system at build time. Modify the Dockerfile as before (or copy the version from the basic/ subdirectory) and add the extra copy line. Once you have done that, build the container like before, but give it a different name:

$ docker build -t alice/alpine-sum .

Exercise: Did it work?

Can you remember how to run a container interactively? Try that with this one. Once inside, try running the Python script.

Solution

You can start the container interactively like so:

$ docker run -it alice/alpine-sum sh

You should be able to run the python command inside the container like this:

/# python3 /home/sum.py

This COPY keyword can be used to place your own scripts or own data into a container that you want to publish or use as a record. Note that it’s not necessarily a good idea to put your scripts inside the container if you’re constantly changing or editing them. Then, referencing the scripts from outside the container is a good idea, as we did in the previous section. You also want to think carefully about size – if you run docker image ls you’ll see the size of each image all the way on the right of the screen. The bigger your image becomes, the harder it will be to easily download.

Copying alternatives

Another trick for getting your own files into a container is by using the RUN keyword and downloading the files from the internet. For example, if your code is in a GitHub repository, you could include this statement in your Dockerfile to download the latest version every time you build the container:

RUN git clone https://github.com/alice/mycode

Similarly, the wget command can be used to download any file publicly available on the internet:

RUN wget ftp://ftp.ncbi.nlm.nih.gov/blast/executables/blast+/2.10.0/ncbi-blast-2.10.0+-x64-linux.tar.gz

More fancy Dockerfile options

We can expand on the example above to make our container even more “automatic”. Here are some ideas:

• Make the sum.py script run automatically:

FROM alpine

COPY sum.py /home
RUN apk add --update python py-pip python-dev

# Run the sum.py script as the default command
CMD python3 /home/sum.py
# OR
# CMD ["python3", "/home/sum.py"]

Build and test it:

$ docker build -t alpine-sum:v1 .
$ docker run alpine-sum:v1

• Make the sum.py script run automatically with arguments from the command line:

FROM alpine

COPY sum.py /home
RUN apk add --update python3 py3-pip python3-dev

# Run the sum.py script as the default command and
# allow people to enter arguments for it
ENTRYPOINT ["python3", "/home/sum.py"]

Build and test it:

$ docker build -t alpine-sum:v2 .
$ docker run alpine-sum:v2 1 2 3 4

• Add the sum.py script to the PATH so you can run it directly:

FROM alpine

COPY sum.py /home
# set script permissions
RUN chmod +x /home/sum.py
# add /home folder to the PATH
ENV PATH /home:$PATH

RUN apk add --update python py-pip python-dev

Build and test it:

$ docker build -t alpine-sum:v3 .
$ docker run alpine-sum:v3 sum.py 1 2 3 4

Key Points

• You can include files from your Docker host into your Docker images by using the COPY instruction in your Dockerfile.

• Docker allows containers to read and write files from the Docker host.