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

Dockerfile

@@ -1,17 +1,17 @@
 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/
 COPY --link symfony.pool.conf ./pool.d/
 
 COPY --from=extension-installer /usr/bin/install-php-extensions /usr/local/bin
 
-RUN install-php-extensions "${PHP_EXTENSIONS}"
+RUN install-php-extensions ${PHP_EXTENSIONS}
 
 RUN usermod -u 1000 www-data && usermod -G staff www-data