prefer-native-arm-runner-over-qemu

Detects Docker image builds that target ARM through QEMU emulation in GitHub Actions.

This rule looks for:

• docker/setup-qemu-action
• a visible Docker build step in the same job
• ARM targets such as linux/arm64

Why it matters:

• Docker documents QEMU emulation as the easiest way to get started, but notes that it can be much slower than native builds for compute-heavy work such as compilation and compression.
• If a workflow mainly builds ARM images, native arm64 runners are often faster and more reliable.
• If a workflow builds both amd64 and arm64, native nodes or per-platform split builds can avoid emulation bottlenecks.

Severity guidance:

• warning: ARM-only builds such as linux/arm64
• suggestion: mixed multi-platform builds such as linux/amd64,linux/arm64

What to do:

• For ARM-only image builds, prefer a native arm64 runner when practical.
• For multi-platform image builds, consider native Buildx nodes, per-platform split builds, or cross-compilation instead of relying on QEMU alone.

This rule is advisory. QEMU can still be the simplest choice when a single multi-platform build invocation is more important than raw build speed.