Fix: Add API_INTERNAL_URL to Dockerfile build stage

Next.js rewrites are evaluated at build time, not runtime.
The API_INTERNAL_URL was set in K8s deployment but not during
the Docker build, causing fallback to localhost:5001.

- Added ENV API_INTERNAL_URL=http://workclub-api:8080
- This ensures Next.js rewrites point to internal K8s service

frontend/Dockerfile

@@ -16,6 +16,8 @@ COPY . .
 # Set environment for build to ensure server binds to all interfaces
 ENV HOSTNAME="0.0.0.0"
 ENV PORT="3000"
+# Set API_INTERNAL_URL for build-time Next.js rewrites evaluation
+ENV API_INTERNAL_URL="http://workclub-api:8080"
 RUN bun run build
 
 # Stage 3: Runtime