[Feature] Add traefik

[jonathan8devs]

With this PR I still want to give the repository the possibility also with Traefik as reverse proxy.

1. added an additional configuration for nginx
2. adapted the docker-compose.yml to version 3.9
3. slightly changed the docker-compose.yml so that the key properties can be made via a docker-compose.override.yml. This preserves the maintenance possibility via Git
4. .gitignore extended by docker-compose.override.yml

[patschi]

I'm okay with supporting different webservers, like traefik in general. But I honestly don't like proposed changes as...

1. It adds another step to set things up
2. Will break all existing settings if they git pull (and previously cloned the repo)
3. The traefik.conf in the nginx folder is slightly annoying, because it wouldn't fit there really

But can't think of any other change to leave everything as-is pretty much and still make it easier for traefik-users...