This is a poor man's version of "only build a container when
the continuous build is green." Travis would be better, but
we can't yet trigger once per commit, only once per build
in the build matrix, so that doesn't work.
The way docker hub and quay.io work, you can't easily pass a custom
context to a Dockerfile. So, effectively, if you want multiple dockerfiles
per repository, you need to re-check out the whole repository by hand,
without help from the environment.
This sucks a little bit because it doesn't guarantee that the build happens
at the trigger revision ID, but for my purposes of "just have a recent build up",
it's sufficient.
