Provide a working Node+yarn example with in-tree Dockerfiles and source

[emk]

Not everybody will have have one git repository per Docker image. For smaller projects, you may just want to keep the Dockerfile and supporting source code directly under src without using an external repo.

To make this work, we need to:

• [x] Add a new subdirectory of examples containing a tiny Node.js project.
• [x] Fix the abs_path plugin to handle relative paths in build:.
• [x] Allow mounting local source directories into containers.
• [x] Rename Repo → Repos and cage repo → cage source.
• [x] Allow not mounting local source directories into containers (aka #18).
• [x] Moved to #27. Figure out what to do about our default .gitignore and the src directory.
• [x] Redesign libraries.yml for our release while we're at it. Maybe as config/sources.yml?

[emk]

@dkastner @seamusabshere cage repo is about to become obsolete as a command name, because we will very shortly support both external source directories and also local source in the main repo.

How do you feel about cage source list, cage source clone myservice, cage source unmount myservice (equivalent to the old mode image) and cage source mount myservice (equivalent to mode repo)?

[seamusabshere]

i love it

[seamusabshere]

"source" and "mount" are exactly the right words

[erithmetic]

source sounds good to me too