docs(dockerfiles): only allow one port exposed

This is due to https://github.com/deis/deis/issues/1156.
chore(controller): update PyYAML to 3.11

http://pyyaml.org/wiki/PyYAML#History
fix(controller): set celery log level from envvar
Merge pull request #1584 from bacongobbler/dockerfile-docs

docs(dockerfiles): only allow one port exposed
Merge pull request #1588 from mboersma/update-pyyaml

chore(controller): update PyYAML to 3.11
chore(client): update docopt to 0.6.2

Minor changes:
https://github.com/docopt/docopt/compare/0.6.1...0.6.2
Merge pull request #1577 from deis/celery-debug

fix(controller): set celery log level to DEBUG
Merge pull request #1590 from mboersma/update-docopt

chore(client): update docopt to 0.6.2
chore(controller): update gunicorn to 19.1.0

http://docs.gunicorn.org/en/19.1/news.html#id1