s6-overlay is a production-grade init system and process supervisor for Docker containers, built on top of the s6 ecosystem (s6, s6-rc, s6-linux-init). It replaces the default container PID 1 with a robust supervision tree that handles zombie reclamation, orderly startup/shutdown, service dependencies, and logging. The architecture is layered: stage0 (s6-linux-init-maker) generates stage1 infrastructure at runtime, stage2 (rc.init) compiles s6-rc service databases and brings up services via s6-rc, supporting both legacy /etc/services.d directories and modern s6-rc source definitions. The system provides utilities like with-contenv (inject container environment), logutil-service (privilege-dropped logging), and with-retries (retry with timeout). It handles edge cases such as read-only root filesystems, non-root USER directives, and Kubernetes world-writable /run configurations. The codebase is predominantly shell and execline scripts, with a Makefile-based build system that cross-compiles all skaware dependencies statically. It is actively maintained, with a clear separation of concerns and extensive documentation derived from the actual code behavior.