Fix bugs and security concerns in docker group API

What does this MR do?

Fixes bugs and security concerns originally raised in gitlab-foss!30817 (comment 204832371), specifically:

• Return container repositories from all nested groups rather than only the specific parent level group.
• Check projects to ensure container registry is enabled before returning their repositories.
• Tighten security by adding additional permission checks throughout the code.
• Refactor the ContainerRepositoriesFinder.

These resolve the discussion points in the mentioned MR that are summarized in #31047 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• 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

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 #31047 (closed)