The FCOS devs introduced a new feature called CoreOS Layering which lets you define your system in a Dockerfile and FCOS will rebase to that state and all you have to do is reboot to configure your server. It is super powerful.
Anyways, your next project needs a VM, give it a shot. I made a Python based CLI tool to help you develop locally on a Linux workstation to create a Butane file to fit your needs. Below is the GitHub for Bupy and a good example of running an app (Paperless NGX) on FCOS with the CoreOS Layering features.
https://github.com/quickvm/bupy
https://github.com/quickvm/fcos-layer-paperless-ngx
https://coreos.github.io/rpm-ostree/container/
https://github.com/coreos/enhancements/blob/main/os/coreos-l...