For hosting multiple web apps, what you probably want is a reverse proxy. I recently started using Caddy (specifically Caddy-Docker-Proxy), and I’m liking it. There’s also Traefik, nginx, etc.

For other types of services, you can simply map whatever (available) port you like in your docker compose file. See here: https://docs.docker.com/compose/networking/

Nextcloud maybe? Not specifically geared toward collaborative music recording, but maybe you could come up with a good workflow.

It’s an interesting use case. I’d be curious to see what you (and/or others) come up with.