If we clone gerrit to ~/src/gerrit.googlesource.com/gerrit but
want to keep the Dockerfile in system-config, then we need to be
able to run:
docker build ~/src/gerrit.googlesource.com/gerrit -f Dockerfile
Most of the time the dir will just be '.', so put in a sensible
default.
Change-Id: I235080c05e679d2ac270cd5401b85c655fab3112