Dockerfile is modified to make image more consistent

Remove obsolete 'base' and use 'ubuntu:12.04' instead of it.

Remove 'apt-get upgrade', because of issues it may introduce.
See details in this discussion: https://github.com/docker/docker/issues/1724

Require ReviewBoard to have version equal to 1.7.24, because sqllite scheme in the GIT repo. (.../data/...) depends on this RB version.