Separate slateci/docker-images into incubabor/stable #131

Suggestion from @muali14 .

The current slateci/docker-images repo and automation scripts make it hard to create a stable and incubator-flavored image of the same name. Maybe we can spit things out into two separate repos and/or modify the existing automation to support this? Also hard in general to tell from the unified list of image source what is being used by stable and incubator app catalogs.