Add support for s390x via "apt-get source --compile"

[tianon]

This takes advantage of the fact that http://nginx.org/packages/ includes the source packages as well, and simply compiles them on non-prebuilt architectures instead, which ensures we have a completely compatible experience with minimal maintenance overhead.

This is based heavily on docker-library/postgres#330, and I've tried to be a little bit over-the-top on comments. 😇

I've personally tested all 4 builds on both amd64 and s390x. 👍

[tianon]

I've currently hard-coded this minimal Architectures list, but as-is, these updates should technically support several other architectures including arm64v8, arm32v7, and ppc64le. For other official images, we usually use bashbrew to scrape the list of Architectures directly from the parent image to embed in the children for cases like this, but I figured that's something we ought to discuss before I implemented it. 😇 ❤️

I'd also be happy to simply adjust this list to include more arches if you'd rather, but then they'll need to be manually updated for any base image changes or new arches (which may be desirable).

I'm also happy to test on any other arches. 👍

[thresheek]

I'm fine with providing images for all the other arches supported - we've seen requests for armv7 and ppc64le at least.

LGTM otherwise.

Thanks!

[tianon]

Alright, neato! I'll have this updated shortly!

[tianon]

I've successfully built stable/stretch on ppc64le, arm64v8, and arm32v7. Happy to do more build tests, but also happy to PR again afterwards if the official images builds happen to fail for some odd reason.

[thresheek]

Many thanks!