Dockerfile-workers: give the master its own log config (#12466)

When we run a worker-mode synapse under docker, everything gets logged to stdout. Currently, output from the workers is tacked with a worker name, for example:

```
2022-04-13 15:27:56,810 - worker:frontend_proxy1 - synapse.util.caches.lrucache - 154 - INFO - LruCache._expire_old_entries-0 - Dropped 0 items from caches
```

- note `worker:frontend_proxy1`. No such tag is applied to log lines from the master, which makes somewhat confusing reading.

To fix this, we generate a dedicated log config file for the master in the same way that we do for the workers, and use that.