XDOCKER-10: Cannot change database name

* Added missing env in the example compose file

docker-compose-using.yml

@@ -34,6 +34,7 @@ services:
     environment:
       - MYSQL_USER=xwiki
       - MYSQL_PASSWORD=xwiki
+      - MYSQL_DATABASE=xwiki
       - DB_CONTAINER_NAME=xwiki-mysql
     # Provide a name instead of an auto-generated id for the xwiki permanent directory configured in the Dockerfile,
     # to make it simpler to identify in 'docker volume ls'.