refactor(controller): optimize layer creation

This restructures the Dockerfile in such a way that allows users to rebuild the
controller with minimal changes to the docker image's layers.
If we're pushing the responsibility for having current `apt-get` cached info to `deis/base`, why not just pass the buck up to `ubuntu:12.04`? (Answer: because you can't rely on cached layers that came before for this purpose.)

This is one reason Docker folks are always preaching "build from source". In this case though I think we can ameliorate the problem by grouping some `apt-get install` commands and adding a couple `apt-get updates` before them. I'll make a comprehensive PR soon--they tell me that's better than whining about it. :smiley_cat: