Changed db build to pre-existing image with command arguments

.examples/docker-compose/insecure/mariadb-cron-redis/fpm/docker-compose.yml

@@ -2,7 +2,8 @@ version: '3'
 
 services:
   db:
-    build: ./db
+    image: mariadb
+    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
     restart: always
     volumes:
       - db:/var/lib/mysql