fix: mount config directory instead of file for Docker Compose

- Add CONFIG_PATH env support to avoid Docker creating config.json as a directory
- Config Save() now auto-creates parent directories
- Update docker-compose.yml to mount ./config:/app/config
- Update README deployment docs accordingly

docker-compose.yml

@@ -1,14 +1,13 @@
 services:
   mirror-proxy:
-    # 使用 GHCR 预构建镜像（推荐）
-    # image: ghcr.io/arkylin/mirror-proxy:latest
-    build: .
+    image: ghcr.io/arkylin/mirror-proxy:latest
     container_name: mirror-proxy
     restart: unless-stopped
     ports:
       - "8080:8080"
     volumes:
-      - ./config.json:/app/config.json
+      - ./config:/app/config
       - ./cache:/app/cache
     environment:
       - TZ=Asia/Shanghai
+      - CONFIG_PATH=/app/config/config.json