Add support for Ubuntu 14.04 (trusty)

Because the Dockerfile starts from ubuntu:latest, it recently started grabbing 14.04 (trusty) instead of 12.04 (precise).

The sources list line was locked to precise, which prevented building because of dependencies.  Also, changes to the PAM configuration of sshd resulted in shell sessions being killed immediately.  This is addressed by making pam_loginuid optional instead of required.

This PR locks the Dockerfile to trusty (which should be "latest stable" for ~2 years) - I'm sure that the 16/V release of Ubuntu will need some manual TLC to get it working, assuming that the world of containers and jenkins hasn't been re-invented by then.