also build trixie and sid base images

I sometimes find myself in a situation where I need a fresh new package from testing or unstable. I think it would be worthwhile, in general, to provide us with a test path to those platforms, particularly during upgrade season (which should really include updating the images here).

So we should really start testing stuff in trixie, if not unstable.

Closes: #15 (closed) Depends on !29 (merged).