Fix reported security issue by Codacy

Author: iazaran

docker/mysql/init.sql

@@ -1,2 +1,4 @@
-ALTER USER 'mvc_user'@'%' IDENTIFIED BY 'mvc_Pass995!';
-FLUSH PRIVILEGES;
+SET @password = '${MYSQL_PASSWORD}';
+
+ALTER USER 'mvc_user'@'%' IDENTIFIED BY @password;
+FLUSH PRIVILEGES;