chore: pin version of Nextcloud to 28.0.3-apache

The `apache` variant was taken because the `latest` tag, which was in
use so far, also matches the `apache` variant.

docker-compose/nextcloud/docker-compose.yaml

@@ -4,7 +4,7 @@ volumes:
   nextcloud-db:
 services:
   nextcloud-app:
-    image: nextcloud:latest
+    image: docker.io/library/nextcloud:28.0.3-apache
     container_name: nextcloud-app
     ports:
       - 80:80