So I decided to work on the single applications (Pi-hole, Nextcloud, Taiga) first.
But that's a good opportunity to learn more about debugging docker.
I switched to start adding the proxy.
Had a look at nginx, it just worked.
But because this is a long-term project, I searched for something more "modern" aka "built for containers".
So I also had a look into traefik. Some hours later I had read through the whole docs, but I only made it happen to run some simple examples, but not traefik + pi-hole + nextcloud in an automaticed way.
Next stop: Caddy