Sorry to revive this, and it may rest with the docker container author, but to shed a bit of light on it, it seems the container hard codes the startup command for the game to have a --port 34197 parameter. So when you run a second instance, when it communicates to the Wube matching server it reports this port (despite asking docker to map it to another port and it comes in/out of your firewall/network at this other port) so when you try to connect to the second instance, it tells the remote client to connect on this hard coded command line port parameter and hits the original server. Is this in your wheelhouse to edit or do I need to contact the container maintainer?
The port to use is not set in the configuration files, it's set from that command line.
Sorry I'm still newish to unRAID/Docker so I don't know if just an environment variable can be added or something.