Add command line option to specify wireguard port

[jeroenhd]

While the default (51820) might work for some, I was already using that port on my network. I was surprised to see that it wasn't possible to change the port Wireguard will be configured to listen on.

This pull request adds a command line option to change the port as well as adding an environment variable for the Dockerfile (SUBSPACE_WIREGUARD_PORT)