update docker file

Author: xianhu

Dockerfile

@@ -45,6 +45,8 @@ ADD ./Dockerfile_requirements.txt /root/
 WORKDIR /root/
 RUN pip3 install --upgrade pip
 RUN pip3 install -r Dockerfile_requirements.txt
+
+# clean everything
 RUN rm -rf /root/*
 
 # change python to python3