Atomic Reactor is a Python library that provides a command-line interface for building Docker container images with enhanced functionality beyond standard Docker build capabilities. The tool enables users to build Docker images within separate Docker containers, providing isolation during the build process. It supports using Git repositories as sources for Dockerfiles, allowing for version-controlled build configurations. Key features include the ability to push built images directly to container registries, collect and manage build logs for analysis, and integrate with the koji build system used in Fedora packaging workflows. The tool can inject arbitrary yum repositories during builds, modify base images in existing Dockerfiles, and retag base images as needed. Atomic Reactor also includes testing capabilities that can be executed after image builds complete, helping ensure image quality and functionality. The tool is designed to work with containerized build environments and supports various container registry operations for image distribution.