perf: build docs on the CI runner with a warm .next cache (#840)

Move `npm run build` out of the Docker image onto the runner, where actions/cache persists .next/cache across runs (the in-Docker build discarded it every time). The image now just packages the prebuilt .next and serves it with `next start`; runtime and the DocSearch entrypoint injection are unchanged.

Also: checkout full history (fetch-depth: 0) so per-page dates are correct, guard buildGitDateMap against shallow clones (correct-or-absent, never wrong), modernise the Docker actions (build-push-action v6, provenance: false), and add a path-filtered pull_request trigger so pipeline changes are validated before merge.

Smoke-tested via isolated build + docker run: serves /, /introduction, /api, sitemap, static assets (200); DocSearch placeholder injection intact.

docker/Dockerfile

@@ -10,17 +10,16 @@ RUN npm install --global pm2
 # Utilise Docker cache to save re-installing dependencies if unchanged
 COPY ./package*.json ./
 
-# Install dependencies (npm ci = clean, reproducible install from the lockfile)
+# Install runtime dependencies (npm ci = clean, reproducible install from the lockfile)
 RUN npm ci --omit=dev
 
-# Copy all files
+# Copy the app, including the prebuilt .next output. The build now runs on the
+# CI runner (see .github/workflows/build_n_push.yml) with a warm .next/cache,
+# so there is no `npm run build` step here — we only package the result.
 COPY ./ ./
 
 COPY /docker/entrypoint.sh ./entrypoint.sh
 
-# Build app
-RUN npm run build
-
 RUN chmod u+x /usr/app/entrypoint.sh
 
 # Expose the listening port