Support Mac

This adds a separate ID passed from the host to the container. This ID passing fixes the lb-su script on a Mac Docker host, where it cannot guess the user from the current directory permission inside the container. It should be backward compatible if you do not pass the IDs to the container, and is more reliable if you do. This fix is intended to help the upcoming Hackathon.

Tested on the latest Docker (1.13) on Mac. Incremented the build tag.