Return ports exposed by the image

Return a list of ports exposed by the base image alongside the user-exposed ports on a service. We want to differentiate between these to preserve the template-defined state.
```
{
    ...
    "expose": [],
    "default_exposed_ports": [
        "6379/tcp"
    ],
    ...
}
```
If we don't need the attribute in the model, we can ditch this too