Not what was mentioned by parent but I've been working on an embedded Linux build system that uses rootfs from container images: https://makrocosm.github.io/makrocosm/

The example project uses Alpine base container images, but I'm using a Debian base container for something else I'm working on.

Nah, first time I’ve mentioned it anywhere. Happy to answer questions, if there’s interest maybe this could be my reason for a first blog post.

Honestly this is just sorta a Tuesday for an advanced Gentoo user? There are lots of ways to do this documented on the Gentoo wiki. Ask in IRC or on the Forum if you can't find it. "Catalyst" is the method used by the internal build systems to produce images, for instance https://wiki.gentoo.org/wiki/Catalyst.