use _GIT_TAG for staging repo image tag

[nojnhuh]

$COMMIT_SHA seems to be empty as the resulting image was tagged with the default v0.1.0 tag: https://console.cloud.google.com/artifacts/docker/k8s-staging-images/us-central1/dra-example-driver/dra-example-driver/sha256:0e8f5df34c196e5900c4d29667ce5b21ba482ef9aecc5dd19cea14fa6dde2d0c;tab=attachments?project=k8s-staging-images

$_GIT_TAG is a variable injected by the Prow job that seems to be what's generally used for image tags: https://github.com/kubernetes/test-infra/blob/3e2b92318e41a47e569a90f0c71db3bcb81b9902/images/builder/README.md

[pohly]

I am unsure how expansion works in the env section. This might have been the problem with the original $COMMIT_SHA, too.

Let's do it as in https://github.com/kubernetes-csi/csi-release-tools/blob/90efb2ca59900f19eba05e65da28beda79c5bb28/cloudbuild.yaml#L32-L47 ?

[nojnhuh]

I've updated this to follow that pattern.

The docs show at least one example of substitutions of these automatic variables without an explicit mapping in substitutions. I think COMMIT_SHA was empty because Prow invokes a "manually triggered" build which doesn't seem to include that variable.

[pohly]

If it can work without the substitutions, then let's try that variant first because it's simpler.

[nojnhuh]

Sounds good, changed this back.

[pohly]

Nit: _GIT_TAG doesn't come from Prow, it's from the builder tool.

Magic.

Let's try it, we can always iterate further.

/lgtm
/approve

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