Implement support for multiple PHP versions

This commit provides docker images and scripts to support multiple
Debian and PHP releases via containers.

stretch_php7/start-fpm.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+set -e
+
+sed "s/@user@/${FPM_USER}/g; s/@variant@/${FPM_VARIANT}/g" \
+    < /usr/local/etc/fpm-pool.conf.tmpl \
+    > "/etc/php/7.0/fpm/pool.d/${FPM_USER}.conf"
+
+/etc/init.d/nullmailer start
+mkdir -p /run/php
+/usr/sbin/php-fpm7.0 --nodaemonize