title: What is the special gitlab-ci-token user?
body: <p>The docs for gitlab's container registry (<a href="https://gitlab.com/help/container_registry/README.md" rel="noreferrer">https://gitlab.com/help/container_registry/README.md</a>) provide a config example with this instruction:</p>
  
  <p><code>- docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN registry.example.com</code></p>
  
  <p>and this explanation:</p>
  
  <p><code>You have to use the credentials of the special gitlab-ci-token user with its password stored in $CI_BUILD_TOKEN in order to push to the Registry connected to your project. This allows you to automated building and deployment of your Docker images.</code></p>
  
  <p>I cannot find any documentation about this special <code>gitlab-ci-token</code> user, nor about the <code>$CI_BUILD_TOKEN</code> var.</p>
  
  <p>What is this special user? Is it automatically available? Does it have to be defined somewhere? What value should be given to the <code>$CI_BUILD_TOKEN</code> var?</p>