fix(Dockerfile): move ARG PHP_EXTENSIONS into the right stage

Dockerfile

@@ -1,9 +1,9 @@
 ARG PHP_VERSION
-ARG PHP_EXTENSIONS
 
 FROM mlocati/php-extension-installer:2 as extension-installer
 FROM php:${PHP_VERSION}-fpm as base
 FROM base
+ARG PHP_EXTENSIONS
 
 WORKDIR /usr/local/etc/php
 COPY --link symfony.ini ./conf.d/