Docker entrypoint does not pass on command line arguments

The Docker entrypoint does not pass command line arguments that are given to `docker run`:

https://github.com/google/copybara/blob/a66f59c5898cda9a3a20ffceecfad1d0f506e456/.docker/entrypoint.sh#L17

This presents a problem for running workflows that accept command line arguments.

Further, I'm unclear why `COPYBARA_*` env vars were preferred over simply doing something like:

```
java -jar /opt/copybara/copybara_deploy.jar "$@"
```

Any pointers much appreciated.

Thanks