Update Dockerfile

Dockerfile

@@ -1,13 +1,12 @@
 FROM python:3.9.0-alpine
 
-RUN pip install -i https://pypi.tuna.tsinghua.edu.cn/simple --upgrade pip && pip install -i https://pypi.tuna.tsinghua.edu.cn/simple mkdocs mkdocs-material
+RUN pip install -i https://pypi.tuna.tsinghua.edu.cn/simple --upgrade pip
+RUN pip install -i https://pypi.tuna.tsinghua.edu.cn/simple mkdocs-material==9.0.2
 
 WORKDIR /app
 
 COPY codes /app/codes
-
 COPY docs /app/docs
-
 COPY mkdocs.yml /app/mkdocs.yml
 
 RUN mkdir ./docs/overrides && mkdocs build