Extend run config to let users specify an IP

Added an option '--ip' to 'docker run' to be able to statically specify the IP for a container. This change helps get around the problem of IP changing across container restarts.
@thockin.
`mustParse` is fatal on error, and does not return. Thus if `--ip 1.2.3` fails to parse, the test immediately bails out. You want regular `parse()` instead so you can test the `err` return value.