Create GH action workflow to merge per-arch images into multi-arch tags

[nickboldt]

Is your task related to a problem? Please describe.

Today, some che images are built multi-arch style w/ qemu + buildx, but this doesn't scale for larger builds like che-theia.

So, in #19688 it's been suggested we build on s390x and ppc64le using travis community hardware.

But then we need to merge these single-arch images into the multi-arch tags that Che operator and registries expect to see when deploying Che and launching workspaces.

Describe the solution you'd like

Therefore we need a GH action that can be triggered when a new Travis build is done, and will append that new image into the shared multi-arch tag.

eg., when :7.32.0-s390x is published, the :7.32.0 tag can be updated with docker manifest --amend to add the new images

          docker manifest create ${{ env.IMAGE }}:multiarch-next $AMEND
          docker manifest push ${{ env.IMAGE }}:multiarch-next

-- https://github.com/eclipse-che/che-dashboard/blob/main/.github/workflows/ci-multiarch.yaml#L62-L71

One caveat to verify: must confirm that something like CACHE_IMAGE_FULL: docker.io/cheincubator/che-dashboard:cache will work for collecting images into a single manifest, eg., https://github.com/eclipse-che/che-dashboard/blob/main/.github/workflows/ci-multiarch.yaml#L62-L71 ... even if they're built w/ different pipelines.

Or do we need a travis cache image and a separate GH action cache? Or none of these, since we're moving to single-arch publishes and a downstream manifest-amender process?

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