chore(server): allow using a FPM socket

roles/server/README.md

@@ -6,3 +6,9 @@ regardless of wether the `apache` or `fpm` docker image is used.
 It provides various common (optimization) configuration options
 and creates a user on the host which is mapped into the container,
 so the host file permissions remain comprehensible.
+
+## Configuration
+
+- `nextcloud_socket_path`: Setting this (to, for example, `{{ nextcloud_basepath }}/socket`),
+  will make FPM listen on `{{ nextcloud_socket_path }}/nextcloud.sock` on the host, enabling
+  you to use FPM to interface with nextcloud.