ci: Mount ssh_known_hosts in rsync container

The problem with having stages that only run in some situations is that
they don't get tested.

Need to mount the SSH host key database file in the _rsync_ container so
the host key of the web server can be verified when publishing.

ci/podTemplate.yaml

@@ -16,3 +16,8 @@ spec:
       signal.pause()
   - name: rsync
     image: git.pyrocufflink.net/containerimages/rsync
+    volumeMounts:
+    - mountPath: /etc/ssh/ssh_known_hosts
+      name: volume-0
+      subPath: ssh_known_hosts
+      readOnly: true