Update CI/CD example documentation to include untagging images

Problem to solve

We have documentation on how to build Docker images using the GitLab Container Registry. However, we do not include how to remove images in the example.

Docs impacted

• 12.3 bloghttps://about.gitlab.com/2019/09/22/gitlab-12-3-released/): image is incorrect
• Building Docker images with CI/CD and the Container Registry -->

Further details

Proposal

Now that we've extended permissions for CI_REGISTRY_USER to be able to untag images, we should update our example documentation.

Who can address the issue

Other links/references