This cuts image size from under 300MB to under 100MB. Plus
we will never actually need this to have additional libraries
installed, since it's talking to the docker socket directly
to build new images. So switching to alpine here is worth it.
- Don't do pip install -U pip - this makes the image build more
non-deterministic than necessary. 17.04 has the latest version of
pip, and we can stick to newest ubuntu quite aggressivley to keep
up in the cases it matters.
- Set --no-cache-dir in pip install, so we don't actually build
wheels - they are useless in our situation. This also lets us not
have to install wheels
- Make note about