CI_REGISTRY should be used to generate CI_REGISTRY_IMAGE
CI_REGISTRY is by default set to the gitlab instance's registry, but it can be overwritten by the user using the secret variables feature. When CI_REGISTRY is overwritten CI_REGISTRY_IMAGE still points to the former registry. It would be much more convenient when CI_REGISTRY is obeyed when generating CI_REGISTRY_IMAGE
Use CI_REGISTRY to generate CI_REGISTRY_IMAGE
This is useful when moving from the integrated registry (e.g. registry.gitlab.com) to another one.