Automatic merge from submit-queue

GCI/Trusty: support the Docker registry mirror

@roberthbailey @zmerlynn please review it.

cc/ @fabioy @dchen1107 @kubernetes/goog-image FYI.

cc/ @ojarjur it is very straightforward to add support for GCI, which is pretty much like the change to ContainerVM's configure-vm.sh in your original PR #25841.