ci: native per-platform Docker builds with cargo-chef and registry cache

[digizeph]

Summary

• Replaced QEMU-emulated multi-platform Docker build with native per-platform builds on AMD64 and ARM64 runners
• Added cargo-chef to Dockerfile for dependency-layer caching
• Switched BuildKit cache from GHA to registry-based cache

Changes

.github/workflows/docker.yml

• Split single QEMU job into a two-entry matrix:
  • linux/amd64 on ubuntu-latest
  • linux/arm64 on ubuntu-24.04-arm (native ARM runner)
• Each build pushes an untagged image by digest
• Added merge job that assembles a multi-architecture OCI image index via docker buildx imagetools create
• Per-platform registry cache (bgpkit/monocle:buildcache-{arch})

Dockerfile

• Three-stage cargo-chef pattern: chef → planner → builder
• Dependencies compiled from recipe (cargo chef cook) in a cacheable layer separate from application source
• RUN --mount=type=cache for Cargo registry

CHANGELOG.md

• Added Build & Infrastructure section

Expected impact

Metric	Before	After (expected)
ARM64 build	~54 min (QEMU emulation)	native compile time
Source-only changes	Full dependency recompile	Only application crate recompiled
Cache budget	10 GB GHA limit	Unlimited (registry)

Verification

• Local Docker build succeeded; binary reports monocle 1.3.0
• actionlint passes
• docker buildx build --check passes (no warnings)
• Repeated local build confirmed all layers cached