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.
