Update to Alpine 3.7

[tobiasge]

Because the tag "python:3.6-alpine" points to Alpine 3.4 which is now
end of life (as of 2018-05-01) the image now pulls the Alpine 3.7 python
image

[calmyournerves]

@tobiasge Seems like the docker-library/python#290 already changed python:3.6-alpine to use alpine3.7, do you think this is still needed?

[tobiasge]

Hello,
yes, I think it is useful to merge this, because the build will be stable even if the tag python:3.6-alpine gets moved again.

[calmyournerves]

You're right, explicit is better!