migration run in their own container

[asters-marnec]

For the self-hosted version it would be ideal if the docker-compose.yaml would also declare a container for running migrations. This container would run before the backend (the backend depends on this). This would eliminate the need for users to manually take care of migrations as stated in the README

[teeohhem]

This is a nice suggestion, thank you! I will talk it over with the team and let you know asap.