I don't know how much it helps, but you can just listen on other ips on localnet, with no need to plumb e.g.

  nc -l 127.0.0.2 9999

Then "all" you need to something to map the 127.0.0.n against the docker instance. This even works on windows by the way. Just start up that second tomcat or whatever with the next 127.0.0.n address and enjoy.