fix: make .next/cache writable in Docker container

The nextjs user couldn't write to .next/cache at runtime because
the directory was created by root during the build stage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -42,6 +42,9 @@ COPY --from=builder /app/.next/standalone ./
 COPY --from=builder /app/.next/static ./.next/static
 COPY --from=builder /app/public ./public
 
+# Ensure .next/cache is writable by the nextjs user
+RUN mkdir -p .next/cache && chown -R nextjs:nodejs .next
+
 USER nextjs
 
 EXPOSE 3000