List all port mappings when no port is specified

Make `PRIVATE_PORT` argument of the `docker port` command optional.

When no port is specified, all ports mapping will be listed, for example:

```bash
$ docker port focused_darwin
7070/tcp
        0.0.0.0:7171
80/tcp
        0.0.0.0:80
        0.0.0.0:81
$ docker port focused_darwin 80
0.0.0.0:80
0.0.0.0:81
```