C is necessary because PHP is written in C. So CGO is used to interface with PHP directly from Go, from a Caddy plugin. Docker is definitely not necessary, but

C is necessary because PHP is written in C. So CGO is used to interface with PHP directly from Go, from a Caddy plugin. Docker is definitely not necessary, but it is the easiest way to ship something that just works. Since you need a bunch of build dependencies to compile PHP, the installation steps are different for every distro to pull those in with whatever's your package manager.