Updated base image to python:2-alpine3.7 (#5889)
Updated base image to python:2-alpine3.7 (#5889)

Updated base image from python:2-alpine to python:2-alpine3.7. Python:2-alpine internally utilises alpine version 3.4 which end-of-life date is the first of May 2018.
lmcro/letsencrypt,stweil/letsencrypt,letsencrypt/letsencrypt,stweil/letsencrypt,lmcro/letsencrypt,letsencrypt/letsencrypt