You are eventually going to be using ruby and related tools and dependent libraries. There are dozens maybe hundreds of ruby versions and interpreters. The system we use to manage this is called 'rbenv'. You need to be able to run rbenv commands from your terminal. Once rbenv is ready you have to install MRI ruby version 2.1.5.
The ruby library system is called 'gem' Since there are many thousands of gems, each with different versions, there are tools to manage this. Like maven. We (and pretty much everyone) use bundler. You specify exactly what dependencies are needed, and bundler installs them.
You often will see commands we share like 'bundle exec rake' -- on a properly setup system this will execute the command 'rake' in the context of the correctly installed dependency libraries. Also on a properly installed system, you wouldn't need to specify 'bundle exec', but we often leave it explicitly there for clarity.
So once bundler is installed, and you've run 'bundle install' you have the correct dependencies -- for ruby type work. You already showed me that you installed the correct docker dependencies. Bundler will install the correct puppet dependencies for you too.