Switch nextcloud database from mysql:latest to fixed major version.

nextcloud/docker-compose.yaml

@@ -79,7 +79,7 @@ services:
       - ${PWD}/nextcloud.conf:/etc/nginx/conf.d/default.conf
 
   db:
-    image: mysql:latest
+    image: mysql:9
     restart: unless-stopped
     volumes:
       - nextcloud-db:/var/lib/mysql