Create "vaporio/python:*-lite" images

[edaniszewski]

This PR creates "lite" images for python which are based off of vaporio/foundation instead of vaporio/buildpack-deps. The goal for this is to be able to use the standard images as the "builder" stage, and the lite images as the "final" stage, where appropriate (see: https://github.com/vapor-ware/synse-server/blob/6eef553aa3ce4ec9bfce48d4e17a5ee259d5ca3a/dockerfile/synse.dockerfile)

This should help make production images leaner,

REPOSITORY              TAG              IMAGE ID         CREATED          SIZE
vaporio/synse-server    3.6-lite         0c62e927c432     16 hours ago     286MB
vaporio/synse-server    3.6              653f5d749538     16 hours ago     848MB

Often, these images (both the new *-lite images and the previous normal images) fail to build in CI because of gpg keyserver issues, which seems like it is a fairly common problem (docker-library/official-images#4252 (comment)).

I've been able to build the images locally repeatedly just fine, but not in CI. Solutions range from adding in retries to the CI build or trying a bunch of different keyservers until one succeeds, not sure which approach makes sense, but it seems like we'll want at least one of them if we want the builds to complete reliably.

[edaniszewski]

@marcoceppi

In my latest commit I added a bit to the bionic builds to try hitting different gpg keyservers on error to try and make the builds more fault tolerant. It looks like it was the final nudge needed to get the builds to work. Not sure if its the "correct" thing to do, so I'll defer to your judgement on that, but it works.

FWIW, this is a known issue and there are a bunch of issue threads on various official image repos (see: docker-library/php#586 (comment)). The TL;DR seems to be that they know its an issue but don't want to update every one of the official dockerfiles to add this kind of thing in.

🤷‍♀️

[marcoceppi]

Yeah, I was watching them build but lost track of time. This looks great I'll squash this down - thanks for chasing on this

[marcoceppi]

Available in master 065e657