Use dynamically mapped ssh port number

[jay7x]

This is to continue the discussion started in PR #68.

Another thing I wonder about is whether Docker can't do this natively. So create the container and then inspect to find out whatever Docker assigned to it, but I guess we can leave that for another day.

From a brief dig through the docker docs, if you want to use a specific port, you need to map it. I do like the idea of telling it that you want a mapping and letting it decide, but I don't think I've ever seen that done anywhere.

I did few quick experiments with podman w/o explicit ssh port mapping and it was successful. We need to ensure it works ok with real docker and maybe with nerdctl too.

Another question is how to check with Docker Swarm.