Update Dockerfile to use Ubuntu superbuild base

[AlexanderGetka-cbica]

Changes the Dockerfile to point to the newly-created Ubuntu16.04 based superbuild. There's one or two things that would ideally be fixed at a later date, but the Superbuild is usable in its current state. Note that this currently doesn't do specific PR build checking -- the container just clones master and builds that, giving us up-to-date images on Docker Hub.