Run php-fpm as different user (#1745)

php-fpm/Dockerfile

@@ -759,7 +759,14 @@ RUN apt-get clean && \
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* && \
     rm /var/log/lastlog /var/log/faillog
 
-RUN usermod -u 1000 www-data
+# Configure non-root user.
+ARG PUID=1000
+ENV PUID ${PUID}
+ARG PGID=1000
+ENV PGID ${PGID}
+
+RUN groupmod -o -g ${PGID} www-data && \
+    usermod -o -u ${PUID} -g www-data www-data
 
 # Adding the faketime library to the preload file needs to be done last
 # otherwise it will preload it for all commands that follow in this file