Update docker run args invocation for new version

When running ```dokku rethinkdb:start [whatever]``` the script currently spits out the following warning:

```
Warning: '-name' is deprecated, it will be replaced by '--name' soon. See usage.
```

This is the offending line:
```bash
docker run -u $(id -u $RUNAS_USER) -v $VOLUME -name=$CONTAINER_NAME \
```

It runs correctly, but just to be safe I updated it.