fix: replace mysql-server with mariadb-server for Debian 12/13 compatibility

- Use mariadb-server instead of mysql-server (works on both Ubuntu and Debian)
- Update mysql_service to mariadb for proper service management
- Update docker/Dockerfile and start-services.sh for MariaDB

docker/start-services.sh

@@ -1,8 +1,8 @@
 #!/bin/bash
 # Services für LEMP-Stack starten
 
-# MySQL starten
-service mysql start
+# MySQL/MariaDB starten
+service mysql start || service mariadb start
 
 # PHP-FPM starten  
 service php8.4-fpm start