Merge pull request #1541 from robinro/docker-docu-port

add comment: ports need to be exposed.

cloud/docker/docker.py

@@ -62,7 +62,8 @@ options:
       - List containing private to public port mapping specification. Use docker
       - 'CLI-style syntax: C(8000), C(9000:8000), or C(0.0.0.0:9000:8000)'
       - where  8000 is a container port, 9000 is a host port, and 0.0.0.0 is
-      - a host interface.
+      - a host interface. The container ports need to be exposed either in the 
+      - Dockerfile or via the next option.
     default: null
     version_added: "1.5"
   expose: