perf: build docs on the CI runner with a warm .next cache

[SunsetDrifter]

Why

Every deploy cold-recompiled all ~314 pages inside the Docker image, because .next/cache (the Next incremental build cache) can't persist across in-Docker builds on ephemeral CI runners. Moving the build onto the runner lets actions/cache keep that cache warm between runs.

Fourth of the docs build/deploy speed series (Phase B). Foundation + partial win — pairs with the standalone slim-image PR (Phase C) for the full deploy-time drop.

What

• build_n_push.yml: build on the runner — checkout@v4 + fetch-depth: 0 → setup-node + cache: npm → npm ci → actions/cache for .next/cache → npm run build. Then package via build-push-action@v6 (provenance: false to keep a single-arch manifest). Modernized metadata-action@v5 / login-action@v3. Added a path-filtered pull_request trigger so pipeline changes are build-validated (no push) before merge.
• Dockerfile: dropped the in-image npm run build; the image now packages the runner-built .next and serves via next start. Runtime + entrypoint DocSearch injection unchanged.
• .dockerignore: ship the built .next output, exclude only .next/cache; removed inline-comment ambiguity.
• git-dates.mjs: guard buildGitDateMap() against shallow clones (correct-or-absent dates, never wrong). With fetch-depth: 0 the per-page "Updated" dates now populate correctly in prod (previously blank).

Testing

Isolated container smoke test (built .next outside Docker → docker build with the new Dockerfile → docker run → probed):

• /, /introduction, /api, /sitemap.xml, /llms.txt → 200; static CSS → 200
• Container boots (Next.js 16.1.6, ready 354ms); no npm run build inside the image
• DocSearch env-injection intact: no leftover APP_ placeholder, injected value present in static chunks

Notes

• Image is still ~590 MB (push/pull unchanged) — the standalone slim image is the follow-up PR.
• Merging runs the new pipeline + pushes an image; it does not deploy. Verify with a manual prod-deploy afterward.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 095d27cf-e055-4d78-b697-c613d7eb4e7a

📥 Commits

Reviewing files that changed from the base of the PR and between 51afd0c and ab60067.

📒 Files selected for processing (4)

• .dockerignore
• .github/workflows/build_n_push.yml
• docker/Dockerfile
• scripts/git-dates.mjs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/build-on-runner

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