Software containers, such as Docker, are recently considered as the mainstream technology of providing reusable software artifacts. Developers can easily build and deploy their applications based on the large number of reusable Docker images that are publicly available. Thus, a current popular trend in industry is to move towards the containerization of their applications. However, container-based projects compromise different components including the Docker and Docker-compose files, and several other dependencies to the source code that is combining different containers and facilitating the interactions with them. Similar to any other complex systems, container-based projects are prone to various quality and technical debt issues related to different artifacts: Docker and Docker-compose files, and regular source code ones. Unfortunately, there is a gap of knowledge in how container-based projects actually evolve and are maintained.

In this paper, we address the above gap by studying refactorings, i.e., structural changes while preserving the behavior, applied in open-source Docker projects, and the technical debt issues they alleviate. We analyzed 68 projects, consisting of 19,5 MLOC, along with 193 manually examined commits. The results indicate that developers refactor these Docker projects for a variety of reasons that are specific to the configuration, combination and execution of containers, by leading to several new technical debt categories and refactoring types compared to existing refactoring domains. For instance, refactorings for reducing the image size of Dockerfiles, improving the extensibility of Docker-compose files, and regular source code refactorings are mainly associated with the evolution of Docker and Docker-compose files. We also introduced 24 new Docker-specific refactorings and technical debt categories, respectively, and defined different best practices. The implications of this study will assist practitioners, tool builders, and educators in improving the quality of Docker projects.