setup 'php-fpm' dockerfile to install extension PDO

docker-compose.yml

@@ -19,7 +19,9 @@ services:
       - '3306'
 
   php-fpm:
-    image: php:7-fpm
+    build:
+      context: .
+      dockerfile: php-fpm.dockerfile
     container_name: php-fpm
     depends_on:
       - mariadb

php-fpm.dockerfile

@@ -0,0 +1,2 @@
+FROM php:7-fpm
+RUN docker-php-ext-install mbstring pdo pdo_mysql