Fixing link to Dockerfile reference

I was browsing your docs and found the original Dockerfile reference link wasn't
correct (i.e. it was/is going to a 404/search page). I corrected the link here.

rails.html

@@ -30,7 +30,7 @@ ADD Gemfile /myapp/Gemfile
 RUN bundle install
 ADD . /myapp
 </code></pre></div>
-<p>That&#39;ll put our application code inside an image with Ruby, Bundler and all our dependencies. For more information on how to write Dockerfiles, see the <a href="https://www.docker.io/learn/dockerfile/">Dockerfile tutorial</a> and the <a href="http://docs.docker.io/en/latest/use/builder/">Dockerfile reference</a>.</p>
+<p>That&#39;ll put our application code inside an image with Ruby, Bundler and all our dependencies. For more information on how to write Dockerfiles, see the <a href="https://www.docker.io/learn/dockerfile/">Dockerfile tutorial</a> and the <a href="http://docs.docker.io/en/latest/reference/builder/">Dockerfile reference</a>.</p>
 
 <p>Next, we have a bootstrap <code>Gemfile</code> which just loads Rails. It&#39;ll be overwritten in a moment by <code>rails new</code>.</p>
 <div class="highlight"><pre><code class="text language-text" data-lang="text">source &#39;https://rubygems.org&#39;