Draft: Use consistent host for container registry across the documentation

What does this merge request do and why?

This code update changes the GitLab registry host from gdk.test to registry.test. This change is reflected in various commands, such as curl and docker, which are used to interact with the registry. Additionally, instructions for configuring the GitLab Docker runner and minikube to work with the new registry host are provided. These changes ensure that the host is consistent across the documentation.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.