expose the port for `dokku run $APP /bin/bash`

Super useful for debugging. So if you run `docker ps`:

```
CONTAINER ID        IMAGE                         COMMAND                CREATED             STATUS              PORTS                      NAMES
b55e977cbcd0        dokku/$APP:latest          "/exec /bin/bash"      6 seconds ago       Up 5 seconds        0.0.0.0:49176->5000/tcp    berserk_stallman
```