Hi @zilexa and welcome to the Caddy community!
There’s nothing to stop you using the Apache or FPM (with nginx) examples as is in the Quick reference guide together with a Caddy container. There’s no compelling reason to replace nginx with Caddy in the FPM example, other than it’s one less container to deal with.
If you choose to replace nginx with Caddy in the FPM example, then I suspect all you need to do is add the reverse proxy construct to this example’s Caddyfile.
If you choose to use the unadulterated FPM/nginx or Apache examples from the quick reference guide, and use Caddy just for the reverse proxy function, there are a couple of extra lines that you will need to include along with the reverse proxy construct in the Caddyfile. Refer to the Case study: Variations in Caddyfile design for different Nextcloud builds for further clues.