Remove sed stuff from docker-serve.sh

[TuringTux]

Currently, there is a part of docker-server.sh (only active when MUESLI_TESTMODE is active) that creates muesli.yml and alembic.ini based off the corresponding *.example files by replacing certain lines using sed.

The regular expression is not that easy to read, and all in all, this is also probably not that easy to understand for beginners.

It would be great if we could change this to remove the sed stuff.