Force Capybara browser's scale factor

What does this MR do?

Force the browser's scale factor to prevent inconsistencies on high-res devices

This prevents an issue where running screenshot generators would produce incorrectly cropped images on Retina devices.

Screenshots

The screenshots below showcase the expiration policy settings from the spec/docs_screenshots/container_registry_docs.rb generator.

Before	After

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

/cc @nmezzopera