Fix container_registry url for relative urls

What does this MR do?

This MR fixes the temporary path_generator utility meant to find the container registry controller URL to take in account relative_url_root too (so that it will work for self managed instances on relative urls too)

Once again this utility is meant to go away as soon as the graphql implementation is complete.

Screenshots (strongly suggested)

No visual change

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #288752 (closed)