Add support for docker buildx on linux builds

[snay2]

This ensures that the platform in the image manifest matches the platform of the binaries.

Issue #, if available: #584

Description of changes:

• Modify the Dockerfile to pass through the docker buildx built-in arguments TARGETOS and TARGETARCH
• Remove support for linux/arm and darwin/amd64 images, since they are not used in production workloads. Continuing to build them now would require using a different base image than amazonlinux:2, which does not support those architectures.
• Continue using the older build system for Windows images, since docker buildx in our current GitHub Actions configuration does not support Windows builds.
• Add a comment that explicitly annotating the image OS and architecture may not be necessary in the future

Testing:
Tested with GitHub Actions on my fork, publishing to an ECR Public repo in my personal AWS account. Windows and Linux builds worked successfully, and the manifests are correct.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[brycahta]

Continue using the older build system for Windows images, since docker buildx in our current GitHub Actions configuration does not support Windows builds.

What kinda effort are we looking at to align the build with Windows? I'd rather not maintain 2 different systems

[snay2]

Continue using the older build system for Windows images, since docker buildx in our current GitHub Actions configuration does not support Windows builds.

What kinda effort are we looking at to align the build with Windows? I'd rather not maintain 2 different systems

@brycahta Agreed, but currently it's a big unknown to me. I didn't discover it until yesterday. Have been trying to get a build running on a Windows laptop with buildx but so far no luck.

Documentation on that use case seems kinda scarce, too, oddly. Lots of "cross-compile all your Linux containers from a Windows machine" but very little of "use buildx on Windows to build Windows containers".

Should be possible, but I don't yet have any idea how much work it will be.

[brycahta]

Can you also update docker build commands in BUILD.md? It could also be a good place to add some info regarding Windows using build instead of buildx

[brycahta]

and remove passive voice

🙏 many thanks

[brycahta]

/lgtm