Docker entrypoint script fails when no COPYBARA_OPTIONS flag passed

Ran into this during deployment using a dockerized implementation:

```
+ java -jar /opt/copybara/copybara_deploy.jar '' migrate copy.bara.sky push_examples_private_to_public ''
May 16, 2023 6:42:59 PM com.google.copybara.Main configureLog
INFO: Setting up LogManager
Copybara (Version: Unknown version)
0516 18:43:00.214 ERROR: Invalid subcommand ''. Available commands: [generator, help, info, migrate, onboard, validate, version]
0516 18:43:00.217 ERROR: Try 'copybara help'.
```

Because of the ordering in the `[entrypoint.sh](https://github.com/google/copybara/blob/47ce5ccd468b46bf3a813a0ce3e83ee44fead192/.docker/entrypoint.sh#L17)` script, it assumes there's some value to be passed for the COPYBARA_OPTIONS. Moving COPYBARA_SUBCOMMAND up the order in that script resolves the issue.

Note that we opted to use an ENTRYPOINT in the Dockerfile, so we're deviating a little bit from the existing Dockerfile, but because we wanted to abstract away all the command syntax from our devs and just expose the config options as envvars.