Multiple platform / architecture specific images

[embray]

Per discussion today with @nthiery and @jdemeyer, it would be nice to have other images for testing sage on other OSes--i.e. debian, arch, etc. as well as different architectures (mainly Intel-compatible 32-bit and 64-bit).

Docker doesn't provide a straight out-of-the-box way to do this, and it makes sense. Many of the steps taken in a Dockerfile are very dependent on the base image, up to the near final steps of running git clone sage && cd sage && make.

We could write a sort of Dockerfile.in that contains the common parts, and then have separate files that provide the platform-specific pre-build steps. Finally a simple edit to the docker-images Makefile can build per-platform images.

Unfortunately there's no elegant way to name these alternate images. It would have to be something like "sagemath-debian-sid-amd64" or the like. We should document clearly what the default "sagemath" is based on.