Added volume to mariadb in etherpad fixture

Volume constraint is missing in mariadb service
in etherpad example, so added that so that it mariadb container
does not fail in environments where creating container in
/var/lib/mysql is not allowed for non-root process.

script/test/fixtures/etherpad/docker-compose.yml

@@ -10,6 +10,8 @@ services:
       MYSQL_DATABASE: $DB_NAME
       MYSQL_PASSWORD: $DB_PASS
       MYSQL_USER: $DB_USER
+    volumes:
+      - /var/lib/mysql
 
   etherpad:
     image: centos/etherpad