docs: add optional proxy arguments for Docker build instructions (#12272)

### What problem does this PR solve?

Adds instructions for passing optional HTTP/HTTPS proxy arguments when
building the Docker image.

This helps users behind a proxy to successfully build the RAGFlow Docker
image without modifying the Dockerfile itself.

### Type of change

- [x] Documentation Update

README_zh.md

@@ -302,6 +302,15 @@ cd ragflow/
 docker build --platform linux/amd64 -f Dockerfile -t infiniflow/ragflow:nightly .
 ```
 
+如果您处在代理环境下，可以传递代理参数：
+
+```bash
+docker build --platform linux/amd64 \
+  --build-arg http_proxy=http://YOUR_PROXY:PORT \
+  --build-arg https_proxy=http://YOUR_PROXY:PORT \
+  -f Dockerfile -t infiniflow/ragflow:nightly .
+```
+
 ## 🔨 以源代码启动服务
 
 1. 安装 `uv` 和 `pre-commit`。如已经安装，可跳过本步骤：