Add axis for docker builds

[raydouglass]

This PR adds an axis file for parameterizing out the docker builds.

You can checkout the resulting docker images here: https://hub.docker.com/r/gpuci/cccl/tags

Once this file is merged, I'll configure a gpuCI job to listen to this repo and rebuild images as changes are made.

[raydouglass]

@brycelelbach It doesn't look like installing gcc-6 (for example) overrides the default gcc installed in the parent images. Do you want to export CC and CXX variables pointing to the newly installed gcc/g++? Or update the /usr/bin/gcc symlink?

[brycelelbach]

Hey Ray - good catch. We can do this with update-alternatives.

[brycelelbach]

Looks like a missing newline?