add links to the Dockerfiles as canonical external requirements, add example of building and exposing the docs and simplify the Dockerfile for cacheability

docs/Dockerfile

@@ -1,11 +1,16 @@
 from ubuntu:12.04
 maintainer Nick Stinemates
+#
+#    docker build -t docker:docs . && docker run -p 8000:8000 docker:docs
+#
 
 run apt-get update
 run apt-get install -y python-setuptools make
 run easy_install pip
+#from docs/requirements.txt, but here to increase cacheability
+run pip install Sphinx==1.1.3
+run pip install sphinxcontrib-httpdomain==1.1.8
 add . /docs
-run pip install -r /docs/requirements.txt
 run cd /docs; make docs
 
 expose 8000