feat(docker): Add API Gateway to Docker Compose

- Create Dockerfile for API Gateway
  - Multi-stage build using golang:1.25-alpine
  - Minimal runtime image using alpine:latest
  - Exposes port 8080

- Add API Gateway service to docker-compose.yml
  - Depends on Consul and all core services
  - Environment variables for gateway configuration
  - Port 8080 exposed

- Update SUMMARY.md
  - Add API Gateway to service list
  - Add API Gateway to Docker build instructions
  - Update file structure to include API Gateway Dockerfile

docker-compose.yml

@@ -121,6 +121,34 @@ services:
       - goplt-network
     restart: unless-stopped
 
+  api-gateway:
+    build:
+      context: .
+      dockerfile: cmd/api-gateway/Dockerfile
+    container_name: goplt-api-gateway
+    environment:
+      ENVIRONMENT: production
+      REGISTRY_TYPE: consul
+      REGISTRY_CONSUL_ADDRESS: "consul:8500"
+      GATEWAY_PORT: "8080"
+      GATEWAY_HOST: "0.0.0.0"
+    ports:
+      - "8080:8080"
+    depends_on:
+      consul:
+        condition: service_healthy
+      auth-service:
+        condition: service_started
+      identity-service:
+        condition: service_started
+      authz-service:
+        condition: service_started
+      audit-service:
+        condition: service_started
+    networks:
+      - goplt-network
+    restart: unless-stopped
+
 volumes:
   postgres_data:
     driver: local