Missing `VOLUME`? - docker branch

[szabolcs-szilagyi]

On the docker branch's readme - also in the actual file in the project - the Dockerun.aws.json declares a volume to be mounted:

   "Volumes": [
     {
       "ContainerDirectory": "/var/app",
       "HostDirectory": "/var/app"
     }
   ],

However I failed to find a VOLUME ["/var/app"] line in the Dockerfile, that would allow the above mount to happen.