MySql config file permissions fix

Reason: [Warning] World-writable config file '/etc/mysql/docker-default.d/my.cnf' is ignored

mysql/Dockerfile

@@ -13,6 +13,8 @@ RUN ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone &
 
 COPY my.cnf /etc/mysql/conf.d/my.cnf
 
+RUN chmod 0444 /etc/mysql/conf.d/my.cnf
+
 CMD ["mysqld"]
 
 EXPOSE 3306