bullseye is the latest stable, and this is what we will be using for
Cloud Native GitLab going forward so we should test with this.
Since an upgrade of the operating system may break pre-compiled C
extensions, we now introduce a DEBIAN environment flag that defaults
to the legacy buster install but allows us to roll out a bullseye
image with `debian-bullseye` as the image name prefix.
This refactors our custom image building to utilize GitLab parallel jobs
with a matrix.
This makes it easier to parse what kind of matrix we are going to build
in our docker images. Furthermore instead of splitting the image name,
we can simply pull the versions of the tools from the environment
variables.
The ultimative reason: Dogfooding
