feat(builder): use docker-buildx plugin to build for non-native platforms

[estroz]

Fixes #312

Supersedes #314 , which adds ~14 MB to the binary and a few new deps while this only adds ~3 MB and one new dep.

What

Leverage docker buildx for multi-platform output image builds

Why

See #312

How Tested

Manually

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[sharq1]

@estroz will this get merged? That's a big PR - would be a shame to leave it and let it have more conflicts 😉

[gthomson31]

any changes ? Still issues on M1 builds

[kcq]

Still issues on M1 builds @gthomson31 until we have this fully integrated try using the --image-build-engine and potentially --image-build-arch flags. With --image-build-engine=internal Slim will build the output image without using Docker, which might handle the same use case here (building optimized images on non-native platforms). If the target image's architecture isn't picked up properly the --image-build-arch flag should allow you to set it explicitly.

[gthomson31]

Thank you for the quick response will give that a try out