Multi-arch Docker images

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LinkedDataHub's Docker images are auto-built on Docker Hub and only support the linux/amd64 architecture.
Issues like #149 highlight the need for multi-arch images, however Docker Hub does not support that.

It looks like docker buildx is the modern way to build such images. Since we already have a GitHub workflow that can build images, it should be possible to extend it to build multi-arch images and push them to Docker Hub.

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https://www.docker.com/blog/faster-multi-platform-builds-dockerfile-cross-compilation-guide/

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https://www.docker.com/blog/multi-arch-build-and-images-the-simple-way/