use a small python docker base image

[rainchen]

candidate list:

# ↪  docker-images-sort-by-size  | grep python | sort -k 2                                                                                              Sun Sep 29 09:46:01 CST 2019
# 923MB   python:3                                                          ad01f54ca3a9
# 143MB   python:3.7-slim                                                   338ae06dfca5
# 179MB   python:3.7-slim-buster                                            b5a7c089ece3
# 940MB   python:3.7-stretch                                                b9d77e48a75c
# 98.7MB  python:3.7-alpine                                                 39fb80313465
#    

Refs：

https://blog.realkinetic.com/building-minimal-docker-containers-for-python-applications-37d0272c52f3

https://pythonspeed.com/articles/base-image-python-docker-images/