Remove rule overrides from code quality and container scanning jobs

What does this Merge Request do and why?

We had rules defined in three places: those brought in with the include of the job, our own extends: .rules:code-changes, and those in our local job definition.

rules brought in via extends do not override those found in the included template, so we need to override them directly within the job definition. To save us redefining the .rules:code-changes logic, we can use reference tags (we need to override the rules key explicitly, and YAML anchors don't work across files).

As a bonus, this should stop code_quality and container_scanning from running on merge requests with only docs changes.

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 Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.