Fix ARM64 Docker builds with QEMU emulation and buildx

[wkoszek]

ARM64 Docker builds are failing because we're building on AMD64 hosts without proper cross-platform build support.

Error

The requested image's platform (linux/arm64/v8) does not match 
the detected host platform (linux/amd64/v3)
exec /bin/sh: exec format error

Root Cause

We removed use: buildx in #149 to fix the --load flag error, but buildx is actually required for cross-platform builds. Without buildx, Docker cannot build ARM64 images on AMD64 hosts.

The real issue in #149 was the --load flag, not buildx itself.

Solution

1. Re-add use: buildx
2. Set up QEMU emulation in CI workflow for cross-platform builds
3. Set up buildx properly before GoReleaser runs
4. Ensure buildx doesn't try to use --load (it shouldn't for multi-platform)

Implementation

Step 1: Set up QEMU and buildx in workflow

- name: Set up QEMU
  uses: docker/setup-qemu-action@v3

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

Step 2: Re-add buildx to .goreleaser.yaml

dockers:
  - use: buildx
    goarch: arm64

How It Works

• QEMU provides ARM64 emulation on AMD64 hosts
• Buildx uses QEMU to build ARM64 images
• GoReleaser pushes images directly (no --load)
• docker_manifests combines architectures

References

• Docker setup-qemu-action: https://github.com/docker/setup-qemu-action
• Docker setup-buildx-action: https://github.com/docker/setup-buildx-action
• GoReleaser buildx docs: https://goreleaser.com/customization/docker/