I don't think this forks Caddy. Rather, it is a Caddy plugin: https://github.com/dunglas/frankenphp/blob/main/caddy/caddy....
It uses mainline Caddy: https://github.com/dunglas/frankenphp/blob/main/caddy/go.mod
It uses mainline Caddy: https://github.com/dunglas/frankenphp/blob/main/caddy/go.mod
I know you can using Nix for sure. I think Arch/pacman (Arch Build System) and Gentoo/portage (since it is source-based) also have this concept, but I’ve never used it. No clue about other package managers.
The downside is that package updates will require a recompilation, but you can use binary caches to centralise this slightly.