Improve automation for registry configuration

What does this Merge Request do?

Improves automation and documentation for configuring a registry with GDK. Configuration is now driven through gdk.yml.

In particular, it adds support for configuring a local self-signed registry, which is closer to how a production GitLab installation would work, and more secure than a passwordless or unsigned registry.

The updated documentation outlines the new features and shows how a local registry can be configured in a way that does not require editing vendored CI templates when working on Auto DevOps (otherwise --insecure-registry modifications are required to GitLab source code during development).

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.