can essentially make images off of a stable base (fedora atomic i.e. silverblue/kinoite) and the maintenance burden of upkeeping anything from upstream doesn't really exist for the most part
you can "declare" an image with a containerfile kind of. it's not going to make a Nix enthusiast claim it's reproducible because it's really not. the custom image you make is more about preinstalled packages, services, etc.