Bullseye isn't currently supported on the Pi 4

This massively reduces the size of the final container

Use data folder otherwise a second volume is created that becomes orphaned after a docker-compose down.

Fixes: #548

Previously, I had changed the default value to "", to force
users to set the SECRET_KEY value (the app refuses to start
if SECRET_KEY is empty).

The problem with that is, out of the box, with the default
configuration, the tests also don't run and complain about the
empty SECRET_KEY.

So, a compromise: revert back to the default value "---".
At runtime, if SECRET_KEY has the default value, show a  warning
at the top of every page.