fix: grafana alternate domain doesn't work

the variable is not expanded and the domain name label ends up as a
literal "$DOMAIN".

.env.sample

@@ -47,8 +47,6 @@ ENABLE_BACKUPS=true
 # COMPOSE_FILE="$COMPOSE_FILE:compose.grafana.yml"
 # GF_SERVER_ROOT_URL=https://monitoring.example.com
 # SECRET_GF_ADMINPASSWD_VERSION=v1
-## Seperate domain for Grafana
-#GRAFANA_DOMAIN=grafana.example.com
 #
 ## Single-Sign-On with OIDC
 # COMPOSE_FILE="$COMPOSE_FILE:compose.grafana-oidc.yml"

compose.grafana.yml

@@ -39,7 +39,7 @@ services:
         - "traefik.enable=true"
         - "traefik.docker.network=proxy"
         - "traefik.http.services.${STACK_NAME}-grafana.loadbalancer.server.port=3000"
-        - "traefik.http.routers.${STACK_NAME}-grafana.rule=Host(`${GRAFANA_DOMAIN:-$DOMAIN}`)"
+        - "traefik.http.routers.${STACK_NAME}-grafana.rule=Host(`${DOMAIN}`)"
         - "traefik.http.routers.${STACK_NAME}-grafana.entrypoints=web-secure"
         - "traefik.http.routers.${STACK_NAME}-grafana.tls=true"
         - "traefik.http.routers.${STACK_NAME}-grafana.tls.certresolver=${LETS_ENCRYPT_ENV}"