Build and push multi-platform docker image

[radiantly]

This PR adds the publish-docker-image-multiplatform CI job step that pushes a multi-platform docker image when a new tag is created. #1211

Unfortunately, this is not as simple as pushing two images targeting different platforms to the same tag. There are a couple of ways to create a multi-platform image:

• Using docker buildx to build images for all platforms. To do this images targeting non-native platforms would need to be built using QEMU (slower), a remote machine or cross-compilation.
• Building and publishing platform-specific images emscripten/emsdk-x64, emscripten/emsdk-arm64 and then creating a manifest that links to these images. This is the simpler solution to implement, and is what this PR does.

Tested with CircleCI on my fork. See the pushed images at radiantly/emsdk · DockerHub

[sbc100]

Nice! Thanks for working on this.