New version deploy workflow

Has to be automated at some point.

1. Make changes
2. merge into legacy branch
3. create git tag with version number, format until now vYYYY.MM.n
4. write version number into backend/.version using echo \"$(git rev-parse --abbrev-ref HEAD)-$(git describe)\"> ./backend/.version
5. write version number into the image tag in docker-compose.prod.yml
6. docker compose -f docker-compose.prod.yml build
7. docker push ghcr.io/phoenixtwofive/karaoqueue:vYYYY.MM.n
8. update all running containers to the new tag