title: Using BUNDLE_FROZEN
body: <p>I often see the following line in Dockerfiles</p>
  <pre><code>ENV BUNDLE_FROZEN=true
  </code></pre>
  <p>I checked bundler docs and it says &quot;Disallow changes to the Gemfile&quot;.
  But I'm confused why someone would want to disallow people to change Gemfile. After all we may add new gems as a project evolves. Could someone explain why we need this env var, how, and when we use it and why it is often appears in Dockerfiles?</p>