The Docker build process seems to use 'git pull --depth=1', which
doesn't fetch enough history to see a tag, which means tahoe's setup.py
can't compute a version (then 'tahoe --version' reports "unknown").
Pulling a hundred commits should be enough to see a tag without making
the resulting image unnecessarily large. If we go more than 100 commits
between tags, we'll need to increase this, or come up with something
smarter.
It looks like DockerHub's automated builds only have access to the
subtree of the source checkout at+below the Dockerfile. Putting the
Dockerfile in misc/build_helpers/ meant that the build process only had
access to misc/build_helpers/, not the full source tree.
