fix CMD in node image
follow symbolic links for dirname
Easier versioning of Redis Server images.

* add redis-base image
* redis-2.6.15 image builds from redis-base
* make redis log to stdout

Closes #1
add license
visualize docker image dependencies
remove cmd_vagrant
Do not rely on `readlink -f` to find absolute canonicalized filenames.

Mac OS X doesn't support `readlink -f`. Alternatives:
* use greadlink from GNU coreutils
* use perl

Closes #3
add Node/Gulp/Ruby Dockerfile
* fix CMD
* less RUN commands
* add ngr to bin/build_images
* update dependency image