title: How to change the version of Ruby in a Docker image (replace 2.2.0 with 2.0.0 )
body: <p>The Heroku Docker image heroku/ruby installs ruby 2.2.3.</p>
  
  <p>How do I use that image, but use ruby 2.0.0 instead (trying to Dockerize a Rails 3.2 app).</p>
  
  <p>I know that the location of the Heroku buildpack for 2.0.0 is</p>
  
  <pre><code>https://heroku-buildpack-ruby.s3.amazonaws.com/cedar-14/ruby-2.0.0.tgz
  </code></pre>
  
  <p>but cannot see how to modify my Dockerfile so that it will use that version of Ruby instead.</p>
  
  <p>I tried:</p>
  
  <pre><code># Dockerfile
  FROM heroku/ruby
  
  # Install Ruby
  ONBUILD RUN curl -s --retry 3 -L https://heroku-buildpack-ruby.s3.amazonaws.com/cedar-14/ruby-2.0.0.tgz | tar xz -C /app/heroku/ruby/ruby-2.2.0
  </code></pre>
  
  <p>which I'd hoped might overwrite the 2.2.0 with 2.0.0 (keeping the path etc the same) but that command gets ignored when I run <code>docker-compose build</code></p>