[Docker] Rewrite entrypoint

This adds a minimal init process so we get clean signal processing. It also removes anything from the entrypoint except startup of AlekSIS with runuwsgi, as this can now start celery alongside the web handler if so desired.

added part::packaging label

Please resolve the merge conflicts.

added 6 commits

• 97e05eea...ee3c9c24 - 5 commits from branch master
• c82d3215 - Merge branch 'master' into feature/docker-rewrite-entrypoint

Compare with previous version

enabled an automatic merge when the pipeline for c82d3215 succeeds

mentioned in commit 9d2cb0bd

merged