use correct apt-key for test.docker.io/ubuntu

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Add some documentation for the RestartPolicy feature.

Signed-off-by: Jean-Paul Calderone <jean-paul@clusterhq.com>
Merge pull request #8073 from ClusterHQ/7743-restart-policy-api-doc

Add some documentation for the RestartPolicy feature.
Merge pull request #8001 from jfrazelle/fix-key-for-test-release-ubuntu

use correct apt-key for test.docker.io/ubuntu