Provide ONBUIILD variants for easier customization

[yuvipanda]

Without having to understand Dockerfile semantics, it should be possible for end users of these images to:

1. Specify which base image they want
2. Customize it additionally (with environment.yml, postBuild, start, etc)

Based on jupyterhub/repo2docker#487 (comment) and some chat with @jhamman, I think it would be great to have automatically generated -onbuild variants of the images here that support easy customization with environment.yml, postBuild, etc files. They'll only support a subset of what repo2docker supports - for example, since there is no base image with Julia support, REQUIRE files won't work. But this is probably ok for now.