docker/build-push-action needs to be updated in docker-publish.yml

[bradbeck]

docker-publish.yml currently uses a version of build-push-action (docker/build-push-action@ad44023a93711e3deb337508980b4b5e9bcdc5dc) that produces an incorrect digest (docker/build-push-action#569) which results in the inability to verify signed images with cosign.

Updating to a more recent version e.g. docker/build-push-action@v2 appears to resolve the issue.

[Phantsure]

Thank you for bringing this here. I hope someone can raise a PR for same.