Tweak docker bits, dependency handling

[cdunklau]

• Label docker image with commit hash (in Docker Hub automated builds)

• Add explicit-include dockerignore (to avoid sending ~150 MiB to docker if a virtualenv exists in the dir)

• Tell pip to only install the wheels we give it from the build step (it could have grabbed deps if something odd happened)

• Remove current-requirements.txt

  Deps it had pinned are also all pinned in setup.py, it pinned a different (older) Twisted version, and was even missing some explicit dependencies (treq and attrs). Best to just remove it, we can add a lock file after tests are better.

[cdunklau]

Looks like my attempt to label the image with the commit hash doesn't work. Might need to use a build hook?