feat(部署): 支持通过环境变量配置挂载的配置文件路径

- docker-compose.yml 配置文件挂载路径改为 ${CONFIG_FILE:-./config.yaml}
- .env.example 添加 CONFIG_FILE 配置项，方便用户指定自定义配置文件

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

deploy/docker-compose.yml

@@ -29,7 +29,9 @@ services:
       # Data persistence (config.yaml will be auto-generated here)
       - sub2api_data:/app/data
       # Mount custom config.yaml (optional, overrides auto-generated config)
-      - ./config.yaml:/app/data/config.yaml:ro
+      # Set CONFIG_FILE in .env to specify a custom config file path
+      # Default: ./config.yaml (copy from config.example.yaml and modify)
+      - ${CONFIG_FILE:-./config.yaml}:/app/data/config.yaml:ro
     environment:
       # =======================================================================
       # Auto Setup (REQUIRED for Docker deployment)