Add multi-architecture Docker image support (amd64, arm64, arm/v7)

[Copilot]

Docker image only supported amd64. This adds arm64 and arm/v7 support via Buildx multi-platform builds.

• Add docker/setup-qemu-action@v3 for cross-platform emulation
• Add platforms: linux/amd64,linux/arm64,linux/arm/v7 to the build-push step

No Dockerfile changes needed — CGO_ENABLED=0, golang:alpine, and scratch are already multi-arch compatible.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Is support for devices with other cpu architectures being considered?</issue_title>
<issue_description>I found that the image on docker hub seems to only support amd64 as the architecture of the cpu, please consider supporting devices of other architectures? Such as armv7, v7l, v8 architectures?</issue_description>

Comments on the Issue (you are @copilot in this section)

@fffaraz Please consider contributing. PRs are welcome!

• Fixes Is support for devices with other cpu architectures being considered? #3

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[Copilot]

Pull request overview

Adds multi-architecture container image builds to the existing GitHub Actions Docker workflow so the published GHCR image supports amd64, arm64, and arm/v7.

Changes:

• Add QEMU emulation setup via docker/setup-qemu-action@v3.
• Configure Buildx to build/push a multi-platform manifest (linux/amd64, linux/arm64, linux/arm/v7).

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[Copilot]

platforms: linux/amd64,linux/arm64,linux/arm/v7 will make pull_request runs build all 3 architectures under QEMU, which materially increases CI time and can make PR checks slow/flake-prone. Consider using multi-arch only on push (release) builds and limiting PR builds to a single platform (e.g., linux/amd64), and gating the QEMU setup accordingly.