Add the nextcloud cron service

This is necessary for some apps to work, in this instance it is being
enabled for the news app.

etc/nixos/services/nextcloud.nix

@@ -25,6 +25,16 @@
       ];
     };
 
+    nextcloud-cron = {
+      image = "nextcloud:fpm-alpine";
+      entrypoint = "/cron.sh";
+      dependsOn = ["nextcloud-postgres"];
+      extraDockerOptions = [
+        "--volumes-from"
+        "nextcloud"
+      ];
+    };
+
     nextcloud-nginx = {
       image = "nginx:alpine";
       dependsOn = ["nextcloud"];