Implement support for multiple PHP versions

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

stretch_php7/fpm-pool.conf.tmpl

@@ -0,0 +1,15 @@
+[@user@]
+user = @user@
+group = @user@
+listen = /var/run/php-fpm-docker/@user@-@variant@.sock
+listen.owner = www-data
+listen.group = www-data
+pm = dynamic
+pm.max_children = 20
+pm.start_servers = 2
+pm.min_spare_servers = 1
+pm.max_spare_servers = 3
+pm.max_requests = 1000
+chdir = /
+request_slowlog_timeout = 10s
+slowlog = /var/log/php-fpm-docker/@user@-@variant@.slow.log