Changes:
- Dockerfile: Remove multi-stage build, now expects pre-built miroir-proxy-linux-amd64
- Dockerfile: Add inline comment documenting the plan §7 cargo-build template
- CI workflow: Change /workspace/dist → /workspace/artifacts to match plan §7
- CI workflow: Update create-github-release to reference /workspace/artifacts
This aligns with plan §7 and §12: scratch base, no libc, minimal attack surface.
The CI builds the static musl binary separately, then Dockerfile copies it.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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