[Compose] Updated Unbound Tag to 1.15 (Alpine Update)

docker-compose.yml

@@ -2,7 +2,7 @@ version: '2.1'
 services:
 
     unbound-mailcow:
-      image: mailcow/unbound:1.14
+      image: mailcow/unbound:1.15
       environment:
         - TZ=${TZ}
       volumes: