Set Dockerfile to debian:bullseye

The manual-build has broken. See: https://gitlab.wgtn.cat-it.co.nz/mahara/manual-builder/-/jobs/1670041

This is most likely because debian:stable has moved on and broke the rest. So let's pin it to the oldstable version.

Note: this should be moved forward to the current stable version as a matter of course. But maybe it should also be changed to use a ubuntu version instead