Add linux/arm64 architecture support to Docker image

[barryw]

Summary

The current Docker image only supports linux/amd64 architecture. On Apple Silicon Macs and ARM-based Linux servers/CI runners, this requires Rosetta/QEMU emulation which causes compatibility issues with .NET networking.

Current Behavior

$ docker run --rm ghcr.io/barryw/sim6502:v3.10.0 --version
WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested

When using --backend vice, the emulated .NET HttpClient fails to connect to host services even though network connectivity exists (verified with wget).

Proposed Solution

Build and publish multi-architecture Docker images supporting both:

• linux/amd64
• linux/arm64

This can be achieved with Docker buildx:

# In GitHub Actions workflow
- name: Set up QEMU
  uses: docker/setup-qemu-action@v3

- name: Set up Docker Buildx
  uses: docker/setup-buildx-action@v3

- name: Build and push
  uses: docker/build-push-action@v5
  with:
    platforms: linux/amd64,linux/arm64
    push: true
    tags: ghcr.io/barryw/sim6502:${{ github.ref_name }}

Benefits

1. Native execution on Apple Silicon Macs (M1/M2/M3)
2. Native execution on ARM-based CI runners (GitHub Actions larger runners, AWS Graviton)
3. Eliminates .NET HttpClient issues under emulation
4. Better performance (no emulation overhead)

Related

This issue is related to #7 (AOT binaries). Both solutions address the same root problem - native ARM64 support for local development and CI.