Support build with cloudbuild for gcr

[sanposhiho]

part of #19

guide: https://github.com/kubernetes/test-infra/blob/master/config/jobs/image-pushing/README.md

For the most part, it is almost the same as the example in the guide.

[sanposhiho]

/kind feature

[sanposhiho]

image pushing job will be have been created in kubernetes/test-infra#23567

[sanposhiho]

/label tide/merge-method-squash

[Huang-Wei]

Is this conventional? I noticed some repos used gcr.io/k8s-staging-test-infra/gcb-docker-gcloud.

[sanposhiho]

If your build process is driven by a Makefile or similar, you can use GCB to invoke that. We provide the gcr.io/k8s-testimages/gcb-docker-gcloud image, which contains components that are likely to be useful for your builds.
https://github.com/kubernetes/test-infra/blob/master/config/jobs/image-pushing/README.md#makefile-build-example

So, gcr.io/k8s-staging-test-infra/gcb-docker-gcloud image is for Makefile build, maybe...?

Because simple build example uses gcr.io/cloud-builders/docker (and I followed it.)
https://github.com/kubernetes/test-infra/blob/master/config/jobs/image-pushing/README.md#simple-build-example

[Huang-Wei]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Huang-Wei, sanposhiho

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [Huang-Wei,sanposhiho]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment