Build linux/arm64 docker images

[justjake]

The current docker images (example) are published only for linux/amd64.

On my M1 Macbook Pro, using these images tends to take 10-100x longer than using native ARM64 builds of Emscripten from homebrew. However, Homebrew will randomly upgrade my Emscripten version due to python changing or something.

It would be great to be able to use the EMSDK docker image for stability/reproducibility without suffering from super slow builds on my laptop.

It appears that CircleCI supports ARM, so perhaps all that's needed is adding an extra build variant for Dockerfile-related build jobs?

https://circleci.com/docs/using-arm/

[sbc100]

While I agree we should provide arm64 docker images, if you want a solution that works today you could just install emsdk the normal way on macOS. The emsdk binaries provided via emsdk install latest are built for arm64.