Add generic gpb-docker builder
Use the new builder to build git-personal-builder and gpb-docker.
Include branches associated with the commit
Add the ability to pass secrets to the builder
Secrets are sent as a JSON object in the `secrets` form value. They are
then written as files in a directory, the path of which is provided to
the build script.
Refactor for easier request cleanup
Communicate the response status from the handle() function via a channel
rather than with the return value. Doing so allows for request cleanup
with defer, such as removing the build dir.