Add onshutdown support to runc

This will run any containers in `/containers/onshutdown` on a clean
shutdown.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

pkg/runc/etc/shutdown.d/010-onshutdown

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+# start onshutdown containers, run to completion
+
+if [ -d /containers/onshutdown ]
+then
+	for f in $(find /containers/onshutdown -mindepth 1 -maxdepth 1 | sort)
+	do
+		base="$(basename $f)"
+		/bin/mount --bind "$f/rootfs" "$f/rootfs"
+		mount -o remount,rw "$f/rootfs"
+		/usr/bin/runc run --bundle "$f" "$(basename $f)"
+		printf " - $base\n"
+	done
+fi