feat(docker): use native runner

[TonyRL]

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docs: https://docs.docker.com/build/ci/github-actions/multi-platform/#distribute-build-across-multiple-runners
This greatly reduces build time (both cached and non-cached)

[Copilot]

Pull request overview

This PR migrates the Docker build workflow to use native ARM64 runners instead of QEMU emulation for multi-platform builds, significantly reducing build times for both cached and non-cached builds.

Key changes:

• Introduces a matrix strategy with native runners (ubuntu-latest for AMD64, ubuntu-24.04-arm for ARM64)
• Implements digest-based multi-platform image builds with a separate merge job to create manifest lists
• Migrates DOCKER_USERNAME from secrets to variables and adds dynamic repository name extraction

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
.github/workflows/docker-release.yml	Main workflow refactored to use native runners with matrix strategy, digest-based builds, and a new merge job for manifest creation. Replaces cosign signing with GitHub attestations
.github/workflows/docker-test.yml	Updates cache scope and DOCKER_USERNAME reference to align with the new platform-specific caching strategy

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