Merge pull request #9672 from nextcloud/drone-mysqlmb4-fix-stable13

[stable13] Fix drone mysqlmb4 tests

Author: MorrisJobke

.drone.yml

@@ -919,7 +919,7 @@ services:
       matrix:
         DB: mysql5.5
   mysqlmb4:
-    image: mysql
+    image: mysql:5.7.22
     environment:
       - MYSQL_ROOT_PASSWORD=owncloud
       - MYSQL_USER=oc_autotest