When building the docker image, the 'apt-get' and 'pip install' that are
run inside the docker image will fail in a an environment that uses
Support environments with proxy servers by dynamically propagating
'http_proxy', 'https_proxy', and 'no_proxy' into the docker environment
when running 'apt-get' and 'pip install'
This can not be done with either --env-file or --env as those are
arguments to 'docker run'. The command being run here is 'docker build'
Once Docker 1.9 is widely deployed, we should switch to using the 'ARG'
command which was added in Docker 1.9. Currently Ubuntu 14.04 uses
Docker 1.6. Using the ARG command will be a much cleaner solution.