Use more efficient reloader in backend container

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Django's dev server uses almost no CPU time when idle with the watchman reloader. However this requires the `watchman` binary and `pywatchman` library.

The `watchman` binary is packaged in debian bullseye, but not the current stable release (buster).
I tried copying the binary from a 3rd party image in the Dockerfile. It is ugly but it works:
```
COPY --from=workbenchdata/watchman-bin:latest-buster-slim /usr/bin/watchman /usr/local/bin/
RUN install -m777 -d /usr/var/run/watchman/
```

An alternative solution could be to use `runserver_plus` from django extensions with the `watchdog` reloader. It is less efficient (~2% of idle CPU usage on my machine), but easier to setup (we just need to add `watchdog` to dev requirements).

Start by reviewing the Dockerfile and development requirements, then compare the proposed watchman/pywatchman setup with the django-extensions runserver_plus and watchdog alternative. Done means selecting and configuring a supported reloader for the backend container, with the required binary or dependency available and idle CPU usage checked.

Written by the indexing model from the issue text.