Description of the problem

It might make users' lives easier to mark flatpak-builder’s .flatpak-builder cache directories (which understandably live outside the xdg cache locations) as cache folders via the cachedir spec: https://bford.info/cachedir/

Which basically means putting a CACHEDIR.TAG file at its root.

Once done, programs like tar, restic, borg, and deja-dup will either automatically or with-a-flag ignore your build/cache dirs.