Allows tagging of exposed ports with aliases. These are then used to
provide environment variables in any -link'ed containers.

Docker-DCO-1.1-Signed-off-by: Tim Toomey <tim@toomey.co.nz> (github: tim-mit)
Merge branch '2435-port-aliases' of github.com:tim-mit/docker into 2435-port-aliases

Docker-DCO-1.1-Signed-off-by: Tim Toomey <tim@toomey.co.nz> (github: tim-mit)