Use reusable workflow to build an arm64 container with buildx.

[hturner]

Using the reusable workflow from #252, create a workflow to build an arm64 container with buildx.

This can borrow from @iaine's test workflow that used buildx to build both amd64 and arm64 images.

[iaine]

I have a running workflow that has completed, but do we need to add the platform to it as a tag? Otherwise, we might overwrite the builds?