php: add PHP_INI_DIR

Author: glensc

docker/7.1-cgi.Dockerfile

@@ -12,11 +12,14 @@ LABEL org.label-schema.build-date=$BUILD_DATE \
       org.label-schema.description="Docker For PHP Developers - Docker image with PHP CGI 7.1, and Alpine" \
       org.label-schema.url="https://github.com/phpearth/docker-php"
 
-ENV \
-    # When using Composer, disable the warning about running commands as root/super user
-    COMPOSER_ALLOW_SUPERUSER=1 \
-    # Persistent runtime dependencies
-    DEPS="php7.1 \
+# PHP_INI_DIR to be summetrical with official php docker image
+ENV PHP_INI_DIR /etc/php/7.1
+
+# When using Composer, disable the warning about running commands as root/super user
+ENV COMPOSER_ALLOW_SUPERUSER=1
+
+# Persistent runtime dependencies
+ARG DEPS="php7.1 \
         php7.1-phar \
         php7.1-bcmath \
         php7.1-calendar \