Add buildx-compatible multi-arch Docker build for amd64 and arm64

[dahu33]

Summary

This PR updates the Docker build so the image can be built for both linux/amd64 and linux/arm64 with docker buildx.

This partially resolves #96.

What changed

• build the Go builder stage on $BUILDPLATFORM
• select the appropriate C cross-compiler based on BUILDARCH/TARGETARCH
• keep the same-arch path on the native compiler
• keep the runtime image on debian:bookworm-slim so the PR does not introduce a breaking runtime-base change
• tighten .dockerignore so local binaries, VCS metadata, and editor noise are not sent in the Docker build context

Scope note

This PR only addresses Dockerfile-level multi-arch compatibility.

The arm64/multi-arch behavior comes from the builder-stage and cross-compilation changes. The earlier distroless runtime switch has been reverted because it is an independent breaking change and should not be bundled into this PR.

I could not find the image publish automation in this repository, so this does not update the build-and-push path for the published Docker image. If publishing is handled externally, that external pipeline will still need to build and publish a multi-arch manifest for the arm64 image to appear publicly.

Verification

• docker buildx build --platform linux/amd64 --output type=cacheonly .
• docker buildx build --platform linux/arm64 --output type=cacheonly .

[dahu33]

@Bez625 @AntiD2ta any chance someone from your team could review this PR? Thank you!