DEV Notes

More Docker notes

Here are some more Docker notes.

Consider we want to label each Docker image with a git commit and this way we can know the exact git commit which the Docker image was built from. This can be done this way:

FROM python:3.11.0-slim

RUN set -x && \
    apt-get update && \
    apt-get install -y --no-install-recommends procps

ARG GIT_COMMIT
ENV GIT_COMMIT=$GIT_COMMIT
LABEL GIT_COMMIT=$GIT_COMMIT

WORKDIR /usr/src/app
COPY entrypoint.sh .

ENTRYPOINT ["/usr/src/app/entrypoint.sh"]

The actual git commit can be obtained like this

git log --pretty="%h" -1

NOTE: The big H can be used to get the full git hash value - "%H"".

The whole docker build command will be like this

docker build --build-arg GIT_COMMIT="$(git log --pretty="%h" -1)" -t my_python .

Inspect the label

The label is the part of the image so it can be inspected or used for various filters etc.

docker inspect my_python:latest

...
            "Labels": {
                "GIT_COMMIT": "968cf2a"
            }
...

The filtering can be done by many ways, for example, by jq (if it is installed) or by just a grep etc

docker inspect my_python:latest | jq '.[].Config.Labels'

Entrypoint

The entrypoint.sh script is optional here. It could be anything, for example, python

ENTRYPOINT ["python"]
CMD ["--version"]

In this case any arguments provided to the run command will be populated to python and by default --help will be used.

Here is an example of the entrypoint.sh script

#!/usr/bin/env bash

set -euo pipefail

declare -r DEBUG="${DEBUG:-0}"

echo "Build $GIT_COMMIT"
echo "Debug $DEBUG"

if [ "$DEBUG" = "1" ]; then
  exec python "$@"
else
  exec python -m http.server 8000 --bind 0.0.0.0
fi

The set -euo pipefail is used to trap any errors like missed variables etc, more details on this in other posts.
The declare -r creates a read-only variable, so it is a constant actually (check help declare for more details.
The {SOME_NAME:-0} provides a default value (0 is here, but can be anything).
The exec is used to substitute the shell with the command so python (or anything else) becomes a process with PID=1.
The "$@ represents any input arguments sent to the entrypoint.sh script.

Possible usage

Just dive into the python shell.

docker run --rm --name my_container -it --env DEBUG=1 my_python

Show python version

docker run --rm --name my_container -it --env DEBUG=1 my_python --version

Run the “service”

docker run --rm --name my_container -it -p 8000:8000 my_python

Inspect docker disk space usage

Show the disk space used by containers

docker ps --size

The output is a bit cryptic regarding the size. It shows two values - the size and the virtual size, according the docs the size is the size of a writable layer of each container and the virtual size is the total size of a read-only layers plus the writable layer.

Show disk space used by docker daemon (includes images, containers, etc)

docker system df

NOTE: the default output is pretty generic but -v (verbose) options give a lot more interesting details.

Tags: docker