Drop -onbuild Python base image, fix on 3.6.4
Drop -onbuild Python base image, fix on 3.6.4

Docker -onbuild images are deprecated, since they are not fully
dependable
(https://github.com/docker-library/official-images/issues/2076).

I replaced it with a standard image, based on
https://github.com/docker-library/python/pull/196

I also fixed the version on 3.6.4, since there's an issue with 3.6.5
(https://stackoverflow.com/questions/49627807/cryptacular-is-broken).

Closes #20.

puffinrocks/puffin,loomchild/puffin,loomchild/puffin,loomchild/puffin,loomchild/puffin,loomchild/puffin,puffinrocks/puffin