Fix aggressive prune breaking rollback

In the image prune command --all overrides --dangling=true. This removes
the image git sha image tag for the latest image which prevented
us from rolling back to it.

I've updated the integration test to now test deploy, redeploy and
rollback.

test/integration/docker/deployer/update_app_rev.sh

@@ -0,0 +1,4 @@
+#!/bin/bash
+
+git commit -am 'Update rev' --amend
+git rev-parse HEAD > version