Add MySQL version to the environment variables

The default version of MySQL is 5.7.22.

This change follows the change of MySQL for version 8 and the possible incompatibility with php and phpMyAdmin. see the links:

- https://bugs.php.net/bug.php?id=76243
- https://stackoverflow.com/questions/49948350/phpmyadmin-on-mysql-8-0

docker-compose.yml

@@ -40,7 +40,7 @@ services:
         depends_on:
             - mysqldb
     mysqldb:
-        image: mysql
+        image: mysql:${MYSQL_VERSION}
         container_name: ${MYSQL_HOST}
         restart: always
         env_file: