Oh, neat! Is this the project you're referring to?
Looks like Bazzite is listed as an example derivative image. I've heard good things about that OS from newer Linux users' perspectives. But is ublue something an individual user could personally customize, or more like something a development team or community project would build up from?
The landing page referencing layers and the Open Container Initiative, so is this more like a bootable container using overlay file system drivers?
One attraction I appreciate with Nix is the ability to overlay or overload default software options from base packages, without having to repeat/redefine everything else upstream, e.g. enabling Nvidia support for btop to visualize GPU utilization via a simple cuda flag. Replicating lazy-level-evaluation with something buildkit ARGs would be hectic, so do they have their own Dockerfile/Containerfile DSL?