add GitLab CI for linter

This adds a pipeline for using linters. Only flake8 is used for now.

alpine/flake8 doesn't start a shell but rather execute flake8 directly.
However, GitLab CI seems to start all containers by trying to start a
shell:

```
-c if [ -x /usr/local/bin/bash ]; then
	exec /usr/local/bin/bash
elif [ -x /usr/bin/bash ]; then
	exec /usr/bin/bash
elif [ -x /bin/bash ]; then
	exec /bin/bash
elif [ -x /usr/local/bin/sh ]; then
	exec /usr/local/bin/sh
elif [ -x /usr/bin/sh ]; then
	exec /usr/bin/sh
elif [ -x /bin/sh ]; then
	exec /bin/sh
elif [ -x /busybox/sh ]; then
	exec /busybox/sh
else
	echo shell not found
	exit 1
fi
```

Since flake8 is the entrypoint, it gets passed this long string which it
cannot do anything with (as opposed to the - I guess - expected `sh`).

Instead, let's replace the entrypoint of the docker image so that the
runner directly passes the script to the container.

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>

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