Return ports exposed by the image

Return a list of ports exposed by the base image.
```
{
    "id": "e58be8bd0864788f31a3fcd44e60ea3aa6d0c8dacb43920795391023dd4e515b",
    "virtual_size": 494464613,
    "tags": [
        "centurylink/postgresql:9.3"
    ],
    "exposed_ports": [
        "5432/tcp"
    ]
}
```