Refactor image-builder parse tags only mode for testability

[dekiel]

image-builder code responsible for parsing image tags should be extracted to dedicated function to make a feature of parsing tags only testable.

https://github.com/kyma-project/test-infra/blob/078766d1354a76f4c838768be9075ef980e79d9a/cmd/image-builder/main.go#L783

Acceptance Criteria

• [ ] Extract code for parsing tags only mode to dedicated function
• [ ] Write unit tests for refactored feature