Automatic merge from submit-queue
Prep for continuous Docker validation test
```release-note
Add a test config variable to specify desired Docker version to run on GCI.
```
We want to continuously validate Docker releases (#25215), on GCI. This change
adds a new test config variable, `KUBE_GCI_DOCKER_VERSION`, through which we can
specify which version of Docker we want to run on the master and nodes. This
change also patches the Jenkins e2e-runner with the ability to fetch the latest
Docker (pre)release, and sets the aforementioned variable accordingly.
Tested on my local Jenkins instance that was able to start a cluster with the latest Docker version (different from installed version) running on both master and nodes.
@dchen1107 Can you review?
cc/ @andyzheng0831 for changes in `cluster/gce/gci/helper.sh`, and @ixdy @spxtr for changes to the Jenkins e2e-runner
cc/ @kubernetes/goog-image
48b255d424