perf: parallelize Docker build with multi-stage BuildKit

[valarLip]

Summary

Refactor atom_image Docker build into 5 multi-stage parallel builds using BuildKit.

Problem

Nightly Docker build serially compiles mori (44m) + RCCL (20m) + Triton (65m) + aiter (60m) = ~225 min, exceeding the 180 min timeout.

Solution

Split into independent stages that BuildKit runs in parallel:

base ──┬── build_mori   (44 min) ──┐
       ├── build_rccl   (20 min) ──┤
       ├── build_triton (65 min) ──┼── atom_image (merge + ATOM)
       └── build_aiter  (60 min) ──┘

Expected: ~225 min → ~70 min (max of parallel stages + merge)

Changes

• docker/Dockerfile — split atom_image into base → 4 parallel build stages → final merge with selective COPY --from
• docker-release.yaml — add DOCKER_BUILDKIT=1
• OOT image unchanged

Verification

• docker build --target atom_image succeeds
• pip show mori aiter triton atom all present
• Build log shows 4 stages running in parallel
• Total build time < 90 min

[Copilot]

Pull request overview

Refactors the atom_image Docker build to use BuildKit-friendly multi-stage builds so independent components (mori/rccl/triton/aiter) can be built in parallel, reducing nightly CI build time and avoiding the 180-minute workflow timeout.

Changes:

• Split atom_image into a shared base stage plus 4 independent build stages and a final merge stage.
• In the final stage, selectively COPY --from=... artifacts from builder stages and install ATOM.
• Enable BuildKit for the nightly docker release workflow build step.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
docker/Dockerfile	Introduces parallel multi-stage build and final artifact merge for atom_image.
.github/workflows/docker-release.yaml	Enables BuildKit for docker build to allow parallel stage execution.

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