containers build with backports do not install packages from backports

It seems like containers built from the trixie-backports base image do not actually install packages from backports unless expliclty specified.

This is counter-intuitive to me: it means that I can't build a package using backports just by switching the base image, i'd also need to pass -t trixie-backports to the apt incantation, which makes the build instructions not portable between builds.

i'm not sure how to fix this: by default, debian does not install packages from backports unless explicitly requested, but perhaps there's some apt config we could tweak here to do that.

in the meantime, i think the rust image needs to be built against forky to get the right rust version.