Debug next not found: add diagnostic + use direct node invocation

Replace `npm run build` with `node node_modules/next/dist/bin/next build`
to bypass PATH/symlink resolution issues on Alpine. Also adds a diagnostic
`ls node_modules/.bin/next` so CI logs show whether the binary is present.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -20,7 +20,10 @@ RUN if [ -n "$NPM_TOKEN" ]; then \
 
 ENV NEXT_TELEMETRY_DISABLED=1
 COPY . .
-RUN npm run build
+# Diagnose what's in .bin after install, then invoke next directly via node
+# to bypass any PATH / symlink resolution issues with `npm run`.
+RUN ls node_modules/.bin/next 2>&1 || echo "WARN: next not in .bin" ; \
+    node node_modules/next/dist/bin/next build
 
 # ---------------------------------------------------------------------------
 # 2. Runner — minimal image. Standalone server + static assets only.