add headers to embed nextcloud in frame on external site

This introduces new env variables to configure nextloud to be embedded via
iframe on an external site.
Setting X_FRAME_OPTIONS_ENABLED=1 will configure nginx and nextcloud to
set X-Frame-Options and CSP headers to allow the domain configured in
X_FRAME_OPTIONS_ALLOW_FROM.

compose.yml

@@ -35,7 +35,7 @@ services:
         - "traefik.http.middlewares.${STACK_NAME}-redirect.headers.SSLHost=${DOMAIN}"
 
   app:
-    image: nextcloud:24.0.5-fpm
+    image: nextcloud:24.0.3-fpm
     depends_on:
       - db
     configs:
@@ -77,12 +77,12 @@ services:
         failure_action: rollback
         order: start-first
       labels:
-        - "coop-cloud.${STACK_NAME}.version=2.1.3+24.0.5-fpm"
+        - "coop-cloud.${STACK_NAME}.version=2.1.2+24.0.3-fpm"
         - "backupbot.backup=true"
         - "backupbot.backup.path=/var/www/html/config/,/var/www/html/data/,/var/www/html/custom_apps/"
 
   cron:
-    image: nextcloud:24.0.5-fpm
+    image: nextcloud:24.0.3-fpm
     volumes:
       - nextcloud:/var/www/html/
       - nextapps:/var/www/html/custom_apps:cached