Remove stackbrew prefix on ubuntu images now that they're reasonably up-to-date and stable

Everyone probably wants to run `docker pull ubuntu` before building with this change.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3906 from tianon/unprefix-ubuntu

Remove stackbrew prefix on ubuntu images now that they're reasonably up-to-date and stable