docs: add pinned CMP 170HX container recipe and published image digests

[seanphan]

Summary

• docker/cmp170hx/: the Dockerfile + runtime_tree.py that built the images measured in PixelML/club-170hx (from the Morrowmake recipe, MIT), with every input pinned (CUDA base by digest, engine and wheel by full commit).
• Dockerfile.overlay: the 4-file copy-drafts overlay at c93c274 on top of the 127c6f0 build.
• README with the published digests:
  • ghcr.io/pixelml/sm80vllm@sha256:704cbb8841be7c99bd1d5d6a8136fc118ca22cfc85457ef4a669a07ef957e8ff (tf-learnings-127c6f076)
  • ghcr.io/pixelml/sm80vllm@sha256:f0dbb483b000f85ac66adb6f190c11d1af395914df67a0d9b7e54dfe2258fa86 (tf-learnings-c93c274c8)

Test plan

• Digests are the ones docker push reported for the images that were benchmarked.
• Anonymous pull by digest works: manifest HEAD 200 for both digests (package is public).
• Self-build from this README reproduces a working image (not run; ~1 h build).

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