custom docker network does not allow entering container port in mapping

Hi,

We use custom networks (user defined bridge network) for docker containers for container to container direct connections with name resolution.

One can manually create a user defined bridge network and it is available as an option under network in container settings. That part works great. 

However, if that custom network is selected, when one tries to add a port mapping, there is no option for "container port", but only host port and other fields like name and description.

To reproduce, pick any docker template (preferably one in host mode), switch it to the custom network and add a port mapping. You'll see that the container port option does not exist.

To work around it, one has to select "bridge" first, add the port mappings, and then switch back to the custom network.

Thanks