set up cloudbuild multi-arch builder before use

[BlaineEXE]

Cloud builder's default docker install does not set up a multi-arch builder. It's easy to use docker's buildx tools to create a multi-arch-compatible builder.

Previous build failed with this error:

error: multiple platforms feature is currently not supported for docker driver. Please switch to a different driver (eg. "docker buildx create --use")

Stackexchange and some github user to the rescue:

• https://unix.stackexchange.com/questions/748633/error-multiple-platforms-feature-is-currently-not-supported-for-docker-driver
• https://github.com/WilliamDenniss/multi-arch-demo

[k8s-ci-robot]

Hi @BlaineEXE. Thanks for your PR.

I'm waiting for a kubernetes-sigs 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.

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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.

[wlan0]

/ok-to-test
/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BlaineEXE, wlan0

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