ExposedPort should be a mapping, not a list

According to http://docs.docker.io/en/latest/api/docker_remote_api_v1.6/#create-a-container ,
the API version that introduce ExposedPort parameter, the value of ports should be a dict and not a list.

Note that the value has no effect, only the keys. This is why an empty dict is given as value.
Also when using api 1.4, ports is ignored, this is why docker didn't complain so far.