Couldn't find the project in my browser history or Lemmy saves. I'm pretty sure it was Lemmy though that led me to find a GitHub project similar to OSTree. It sounded like it was maintained by one person and it hasn't been updated in a long time because the author thought it was "done" and they used it frequently.

It was a tool that let them basically create images that could be booted from and it was easy to layer software on top of a base image and I think there were config files similar to Containerfiles but didn't look the same. Don't think it be was "goldboot" either but that might be a little closer to what the project does. I don't think it was something Fedora specific either like bootc.

Update: Found it! It was in the history of a laptop I rarely use (of course). The project is https://github.com/godarch/darch and it does appear to be those things I said: layered, docker-like, bare metal, and OS agnostic.