Thanks!

It seems the LAN option needs to be unchecked for a multiplayer game if you have Docker and are attempting to start a game.

My docker bridge is:

bridge0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=63<RXCSUM,TXCSUM,TSO4,TSO6>
ether 6a:00:02:08:4e:d0
inet6 fe80 ...