Add RUBY_VERSION env

[gwmoura]

To speed up the update, do not you think better to add the ruby version in Dockerfile?

I create an image based your image and when I create other tags, just change ruby version from ENV RUBY_VERSION ruby-2.2.3 to ENV RUBY_VERSION ruby-2.2.2

my repo for details: https://github.com/lememilitar/docker-ruby