Use stand-alone arm64 machine to build/test linux/arm64 images

[csandanov]

Issues:

• Right now we only test amd64 image
• arm64 builds take 4+ hours

Solution:

• Spin up a self-hosted arm64 runner on aws graviton
• Add tests for arm64
• Build/push multi-arch images separately, then create multi-arch manifest https://stackoverflow.com/questions/66337210/is-it-possible-to-push-docker-images-for-different-architectures-separately