add nextcloud cronjob

roles/containers/tasks/nextcloud.yml

@@ -51,3 +51,11 @@
     minute: "2"
     job: "/usr/bin/docker exec -u www-data nextcloud ./occ face:background_job -u {{ CONFIG.NEXTCLOUD_USER }} -t 300"
     user: m
+
+- name: nextcloud preview generation
+  cron:
+    name: nextcloud preview generation
+    hour: "*"
+    minute: "30"
+    job: "/usr/bin/docker exec -u www-data nextcloud ./occ preview:pre-generate
+    user: m