Fixes: #565

Also, split Dockerfile into two stages, so rust
and other build dependencies don't end up in the final image.

Note cryptography has binary wheels for various architectures,
but unfortunately not for 32-bit arm. And, starting from v35.0.0,
cryptography requires rust to build from source.

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.