Add a system containerd

This adds an independent system containerd for running internal
containers.

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

alpine/packages/containerd/etc/init.d/containerd

@@ -0,0 +1,17 @@
+#!/sbin/openrc-run
+
+depend()
+{
+	after docker
+}
+
+start()
+{
+	ebegin "Running system containerd"
+
+	# set ulimits as high as possible
+	ulimit -n 1048576
+	ulimit -p unlimited
+
+	/usr/bin/containerd &
+}