images/gcb-docker-gcloud: enable docker-buildx builds

[LorbusChris]

These changes make the gcb-docker-gcloud image usable as a docker-buildx builder, like so (example from #22444):

steps:
  - name: gcr.io/k8s-staging-test-infra/gcb-docker-gcloud
    entrypoint: /buildx-entrypoint
    args:
    - build
    - --tag=gcr.io/$PROJECT_ID/alpine:$_GIT_TAG
    - --tag=gcr.io/$PROJECT_ID/alpine:latest
    - --platform=linux/amd64,linux/arm64/v8
    - --build-arg=IMAGE_ARG=gcr.io/$PROJECT_ID/alpine:$_GIT_TAG
    - --push
    - .
    dir: .
substitutions:
  _GIT_TAG: '12345'

/cc @spiffxp

[k8s-ci-robot]

Hi @LorbusChris. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[LorbusChris]

Is this intentional?
I had do use dir: . for the build to work with

builder  --project=k8s-staging-test-infra --scratch-bucket=gs://k8s-staging-test-infra-gcb images/gcb-docker-gcloud/

[spiffxp]

I think it doesn't need to be this way but may require changes I don't have time to dig into right now. Better to follow the same convention as everything else

[spiffxp]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: LorbusChris, spiffxp

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 [spiffxp]

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