Add ability to add things to the docker entrypoint using rocker extensions

[tfoote]

Some things that are done in the overlayed image might make more sense to be supported in extending the docker entrypoint.

This would allow you to do things like extend paths, set environment variables, and other things more related to runtime. Also elements baked into the entrypoint can allow greater reuse of rocker produced images.

One possible approach would be to have a generic entrypoint that sources all setup files in a directory where extensions can insert their own elements, using a numbered file for ordering.