Add support for multiple platforms in Docker build

[ananich]

Not just PC, but Mac also

[mar10]

Yes, I am aware of this feature, however

GitHub's standard runners are x86_64, so building for linux/amd64 alone is
fast — it compiles natively with no overhead.
When you add linux/arm64 or linux/arm/v7, GitHub Actions has to emulate those architectures using QEMU.
This is noticeably slower because every instruction is being translated on the fly.
Depending on your image, ARM builds can easily take 3–10x longer than a native amd64 build.

So it is slow (and uses up more action time). I would rather like to modify the workflow to do this only on releases

[ananich]

Make sense. I'd say that Docker images are quite heavy, so it makes sense to build both amd64 and arm64 only on releases.