Sorry for reviving an old conversation @jochen but have you seen this thread:
I also found this on Docker Hub:
https://hub.docker.com/r/trevorj/mopidy-extras/
Looks like it contains the Pi Musicbox front-end and instructions on accessing the audio through the docker container. I have no idea how it will work with a DAC.
There are also containers for shareport-sync, snapcast, and spotify connect.
I’m not sure how all of this would run on a Pi and how the audio switch among all the services, but it is interesting to look into.