This document discusses Source to Image (S2I), an open source tool for building reproducible Docker images from source code. S2I uses builder images containing common build tools and assembles the source code into runnable applications within these builder images. It aims to make the build process easy to use, reproducible, and secure while minimizing build dependencies and layers. The document outlines the S2I build process, demonstrates incremental and layered builds, and describes options for customizing builds.