Python 3.7 Alpine 3.8

[askoretskiy]

Since Alpine 3.8 has been released https://alpinelinux.org/posts/Alpine-3.8.0-released.html, it would be great to have python:3.7-alpine3.8

It contains some nice goodies, like newer OpenSSL.

Probably we would need to wait some days to see an official Alpine 3.8 image.

[jpds]

This is blocked on docker-library/official-images#4501

[tao12345666333]

I have opened a PR #303