This allows us to benefit from helpful things it does for us,
like enabling jemalloc: https://github.com/matrix-org/synapse/pull/8553

We weren't going through `start.py` before, because it was causing some
conflict with our `docker run --user=...` stuff, but it doesn't seem
to be a problem anymore.

Having done this, we won't need to do things like
https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/941
anymore.