Make PRIVATE_PORT optional for `docker compose port`

### Description

"Normal" docker already does this. See the usage for `docker port`:

```
$ docker port --help

Usage: docker port CONTAINER [PRIVATE_PORT[/PROTO]]
```

However `docker compose` differs:
```
$ docker compose port --help

Usage:  docker compose port [OPTIONS] SERVICE PRIVATE_PORT
```

I can look into making a PR if you agree this should be optional.