Access denied when creating subwikis

I am currently trying to setup multiple xwikis inside a docker container. Unfortunately, creating new wikis results in an error (Access denied for user 'xwiki'@'%' to database 'nameofthewiki'). Appearently, the xwiki user does not have the right to create databases.

The issue is also referenced here.

Starting the container with DB_USER=root option does not seem to work for me though. Still the same message that it tries to create the DB with xwiki user.

EDIT: Starting with mysql root worked after actually changing it in the config...