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

Author: nanoninja

Diff for: .env

@@ -11,6 +11,7 @@ NGINX_HOST=localhost
 PHP_VERSION=latest
 
 # MySQL
+MYSQL_VERSION=5.7.22
 MYSQL_HOST=mysql
 MYSQL_DATABASE=test
 MYSQL_ROOT_USER=root

Diff for: 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: