[libc++] Rework docker files for buildbots.

I've been sitting on this change for a while and have been using
it to build the bot images, so it should be upstream.

This re-configures the docker build files to use docker-compose
more heavily. This allows for composing large images with multiple
compilers without invalidating the docker caches.

After this commit I'll quickly switch all the current buildbots
over to a new docker image, followed by another update to add new
compilers