docs(infra): add domain mapping documentation for third-party dns

Author: gracefullight

apps/infra/README.ko.md

@@ -14,6 +14,10 @@ GCP 인프라 프로비저닝을 위한 Terraform 설정입니다.
 - [IAM Service Account Credentials API](https://console.cloud.google.com/marketplace/product/google/iamcredentials.googleapis.com)
 - [Service Networking API](https://console.cloud.google.com/apis/api/servicenetworking.googleapis.com/metrics)
 
+### 도메인 설정
+
+Cloudflare, Route53 등 서드파티 DNS를 사용하는 경우, [Cloud Run 도메인](https://console.cloud.google.com/run/domains)에서 도메인 매핑이 필요합니다.
+
 ## 사용법
 
 ```bash

apps/infra/README.md

@@ -14,6 +14,10 @@ Before running `terraform apply`, enable the following APIs in your GCP project:
 - [IAM Service Account Credentials API](https://console.cloud.google.com/marketplace/product/google/iamcredentials.googleapis.com)
 - [Service Networking API](https://console.cloud.google.com/apis/api/servicenetworking.googleapis.com/metrics)
 
+### Domain Configuration
+
+If you use third-party DNS services (Cloudflare, Route53, etc.), you must map your domains at [Cloud Run Domains](https://console.cloud.google.com/run/domains).
+
 ## Usage
 
 ```bash