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

docs/content/stories/epic2/SUMMARY.md

@@ -230,6 +230,7 @@ This will start:
 - Identity Service (port 8082)
 - Authz Service (port 8083)
 - Audit Service (port 8084)
+- API Gateway (port 8080)
 
 ### Infrastructure Services
 
@@ -423,9 +424,12 @@ goplt/
 │   │   ├── main.go
 │   │   ├── authz_service_fx.go
 │   │   └── Dockerfile
-│   └── audit-service/
+│   ├── audit-service/
+│   │   ├── main.go
+│   │   ├── audit_service_fx.go
+│   │   └── Dockerfile
+│   └── api-gateway/
 │       ├── main.go
-│       ├── audit_service_fx.go
 │       └── Dockerfile
 ├── docker-compose.yml
 ├── docker-compose.dev.yml
@@ -473,6 +477,7 @@ docker build -f cmd/auth-service/Dockerfile -t goplt-auth-service:latest .
 docker build -f cmd/identity-service/Dockerfile -t goplt-identity-service:latest .
 docker build -f cmd/authz-service/Dockerfile -t goplt-authz-service:latest .
 docker build -f cmd/audit-service/Dockerfile -t goplt-audit-service:latest .
+docker build -f cmd/api-gateway/Dockerfile -t goplt-api-gateway:latest .
 ```
 
 ### Docker Compose Files