Santeri Kaariainen developed overlay volume mounts support for the gitlabhq/gitlab-runner repository, focusing on the Docker executor in Linux environments. He implemented overlay mount parsing with recognition of the 'O' mode, ensuring full handling of SELinux labels and propagation options to maintain correct volume semantics and security. Using Go and leveraging his expertise in Docker and Linux volume management, Santeri expanded test coverage to address edge cases and reduce regression risk in CI workflows. He also documented the new behavior to support future enhancements, delivering a robust feature that improves reliability and maintainability for containerized build environments.