I’ve only used Caddy as a reverse proxy in production, but on my development machine I use Caddy with php-fpm. That makes me a bit unsure if I understand your questions correctly.

For me that would look something like this:

test.example.com {
	root * var/www/html
	php_fastcgi unix//run/php/php7.4-fpm.sock
	file_server
}

(Yeah, PHP 7.4, I know)

It looks like your Docker (?) image is exposing the php-fmp socket? I did not even know that was possible, but I don’t doubt it is.

Caddy has no issues serving multiple hosts from the same server, it can even be with different php-fpm sockets. Caddy will just nod at you, maybe silently question your choice of still running PHP 7.4, but it accepts it and runs. Just make another block with a different host in the same Caddyfile, and it will work just fine.

I found my answer here at stackoverflow and it worked

basically, it needs to tell the absolute path of php-fpm with the root directive insde php_fastcgi

localhost:80 {
    root * /srv/www
    encode gzip
    php_fastcgi php:9000 {
        root /var/www/html
    }
    file_server
    log
}

then, i had to take care of permissions, because even if caddy is run as root, php-fpm is run as www-data , so the directory ownership need to be assigned to that