Use the user's ssh key automatically in ssh examples

This requires moby tool update to support `~` in paths, but
makes everything much nicer.

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

projects/okernel/examples/okernel_simple.yaml

@@ -18,4 +18,6 @@ services:
     image: "linuxkit/sshd:abc1f5e096982ebc3fb61c506aed3ac9c2ae4d55"
 files:
   - path: root/.ssh/authorized_keys
-    contents: '#your ssh key here'
+    source: ~/.ssh/id_rsa.pub
+    mode: "0600"
+    optional: true