Dockerfile: git-pull more history, to get a version string

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.