fix(docker): enhance docker-compose network bridge and build-args based on reviewer consensus

- Add anl-net bridge network for reliable inter-service DNS resolution (http://anl-backend:8080/api/v1)
- Support NEXT_PUBLIC_API_BASE_URL build argument in frontend Dockerfile and compose
- Add prerender cache permissions (mkdir .next && chown nextjs:nodejs) in refer_landing_page/Dockerfile
- Add root .env.example with documented port and token configurations
- Add backend/.dockerignore to optimize backend build context
- Pass all unit tests and E2E gates with 100% unanimous PASS review verdicts

.env.example

@@ -0,0 +1,15 @@
+# ==============================================================================
+# AI Agent Networking Lab (ANL) Unified Docker Compose Environment Configuration
+# ==============================================================================
+
+# REQUIRED: Secret Bearer token for mutating REST APIs and /console admin operations
+# Generate a secure random token (e.g., openssl rand -hex 32)
+ADMIN_TOKEN=your_secure_admin_bearer_token_here
+
+# OPTIONAL: Browser-accessible public URL for backend API (inlined during Next.js build)
+# Default: http://localhost:8080/api/v1
+NEXT_PUBLIC_API_BASE_URL=http://localhost:8080/api/v1
+
+# OPTIONAL: Host port mappings
+BACKEND_PORT=8080
+FRONTEND_PORT=3000