Automatic merge from submit-queue

Use HOSTNAME in Docker build image tag hash

Fixes #24661 by including `$HOSTNAME` when generating the build image tag hash.

When running the verification checks under Docker, the `$KUBE_ROOT` will be identical across builds, so tags will collide unless we add additional uniqueness. By default, the hostname inside a Docker container is its ID, which should be unique enough for us.

I also deleted a misleading error message from the same check.

@kubernetes/sig-testing