An image with official ruby-2.2.2 as base and phantomjs installed created for the purpose of running Capybara feature specs that use phantomjs.
latest - which has ruby-2.2.2 2.2.1 - which has ruby-2.2.1 2.2.0 - which has ruby-2.2.0
In your Dockerfile, just do:
FROM coders51/ruby-phantomjs:latest