Allow devstack to build ironic agent ramdisk

This provides a way to test Ironic-python-agent code that is
not yet merged.

Change-Id: I85084c383263173a33250592a95a17f46827b347
diff --git a/files/apts/ironic b/files/apts/ironic
index 8674d9f..283d1b2 100644
--- a/files/apts/ironic
+++ b/files/apts/ironic
@@ -1,3 +1,4 @@
+docker.io
 ipmitool
 iptables
 ipxe