Creating More Complex Container Images
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
Let’s create a file and folder called it foo/dummy.py in the root
folder.
Please copy the Dockerfile and place it in the foo directory.
Let’s say we wanted to try running the script using our recently
created ubuntu-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/ubuntu-python python3 dummy.py
Output
python3: can’t open file ‘dummy.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/ubuntu-python python3 dummy.py
But we get the same error!
python3: can't open file 'dummy.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 dummy.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/dummy.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.
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 as some that we’ve finished with our dummy.py script and want to add it to the container itself.
In your shell, you should still be in the dummy folder in the docker-intro folder.
pwd
Output
/Users/yourname/foo
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 dummy.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/ubuntu-dummy .
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/ubuntu-dummy sh
You should be able to run the python command inside the container like this:
/# python3 /home/dummy.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.
More fancy Dockerfile options
We can expand on the example above to make our container even more “automatic”. Here are some ideas:
Make the dummy.py script run automatically:
FROM ubuntu:18.04
COPY dummy.py /home
RUN apt update && apt install -y python3
# Run the dummy.py script as the default command
CMD python3 /home/dummy.py
# OR
# CMD ["python3", "/home/dummy.py"]
Build and test it:
docker build -t ubuntu-dummy:v1 .
docker run ubuntu-dummy:v1
Make the dummy.py script run automatically with arguments from the command line:
FROM ubuntu:18.04
COPY dummy.py /home
RUN apt update && apt install -y python3
# Run the dummy.py script as the default command and
# allow people to enter arguments for it
ENTRYPOINT ["python3", "/home/dummy.py"]
Build and test it:
docker build -t ubuntu-dummy:v2 .
docker run ubuntu-dummy:v2
Add the dummy.py script to the PATH so you can run it directly:
FROM ubuntu:18.04
COPY dummy.py /home
# set script permissions
RUN chmod +x /home/dummy.py
# add /home folder to the PATH
ENV PATH /home:$PATH
RUN apt update && apt install -y python3
Build and test it:
docker build -t ubuntu-dummy:v3 .
docker run alpine-dummy:v3 dummy.py