Please add URL_PREFIX

[soichih]

It looks like graphite 0.10 (finally) supports URL_PREFIX (via -e option?) so that I can run graphite-web UI on non-root location. Please add support for this option with docker-graphite!

[nickstenning]

Unfortunately graphite 0.10 is still not published to PyPI so this would involve a substantial rewrite of the image at the moment. I don't have time to do this right now, sorry.