allow user to specify a single port with default command

[minrk]

Allows specifying -p9999 or -p5555:9999 without also needing to override the default command

app retrieves port arguments from length-1 port mapping for default command, instead of requiring random port OR overriding the whole command.

[minrk]

Notable difference from the docker run -p arg: specifying just 8000 means exposing container port 8000 as host port 8000 rather than container port 8000 as random host port. I think this makes more sense in a repo2docker context, but it is still a deviation from the upstream argument that this inherits from.

[consideRatio]

I rebased on this on current main branch and stashed away the previous state in my fork!