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.
It's not a shell script, but it has makefile rules that make it relatively simple to build a Docker image for your architecture, export it and turn into a filesystem image, build a kernel, u-boot, etc
The referenced "example project" repo builds a basic Alpine image for the Raspberry Pi (https://github.com/makrocosm/example-project/tree/main/platf...) and others
It was motivated by frustrations with Yocto at a new job after 8 or so years working on firmware for network equipment using an offshoot of uClinux.
Hoping to convince new job to use Makrocosm before we settle on Yocto.
The example project uses Alpine base container images, but I'm using a Debian base container for something else I'm working on.