roles: Make domain global variable

Each role has their own subdomains which can be set manually, or can
inherit from the global domain with a default prefix.

roles/grocy/templates/docker-compose.yaml.j2

@@ -13,7 +13,7 @@ services:
       - "{{ arrstack_env_dir }}/config/grocy:/config"
     restart: unless-stopped
     labels:
-      caddy: "http://house.pichi.berlin"
+      caddy: "{{ stack_grocy_subdomain }}"
       caddy.reverse_proxy: "{{ '{{' }}upstreams 80{{ '}}'}}"
 
 networks: