use a msft compliant image (#602)

Dockerfile

@@ -1,5 +1,5 @@
 # basic setup
-FROM python:3.8
+FROM mcr.microsoft.com/mirror/docker/library/python:3.8-buster
 RUN apt-get update && apt-get -y update
 RUN apt-get install -y sudo git
 
@@ -21,4 +21,4 @@ RUN pre-commit install
 
 # override default image starting point
 CMD /bin/bash
-ENTRYPOINT []
+ENTRYPOINT []