Now that we have an initial set of production images available (T372602), we can move forward with preparing 8.1-based image variants for Shellbox.
From a quick cross-check between packages installed by the existing service variants in [0] and what's available in bullseye (which we'll be migrating to as part of this effort), I'm cautiously optimistic.
There are one or two packages (at least among those that use semver) that will require a closer look at what's "breaking" (e.g., fluidsynth), and at least one package where we'll need to prepare a backport build.
Alas, I'm not aware of a way to express "parameterized" variants (e.g., matrix-build like behavior) in blubber, so there's likely to be quite a bit of duplication while we're in a state where both 7.4 and 8.1 are built.