s6-overlay is a process supervisor and init system for containers that replaces the default Docker entrypoint with s6 as pid 1.
The tool solves the problem of running multiple services reliably within a container while maintaining proper initialization and shutdown sequences. Docker's default behavior of running a single process as pid 1 leaves no room for process supervision, graceful shutdown, or service management. s6-overlay addresses this by providing a complete init system based on s6, a lightweight supervision suite. Installation is straightforward: extract a tarball into your Docker image, and the overlay integrates with your existing setup without requiring image modifications. The system handles multiple init stages, service supervision with automatic restart, logging with log rotation, privilege dropping, and graceful shutdown of all managed services.
The tool suits projects that need to run multiple interdependent services in a single container or require robust process supervision and ordered startup and shutdown sequences. It works on top of any existing Docker image without requiring changes to the base image itself. The README emphasizes compatibility with previous versions and provides migration guidance for users upgrading from earlier releases, indicating that the project maintains backward compatibility as a priority.
Development activity shows consistent maintenance with attention to compatibility across container architectures and careful documentation of upgrade paths. The project provides detailed release notes and upgrade guidance for users migrating between major versions. Build verification is automated, and the maintainers support multiple target architectures with explicit guidance on architecture selection. The project includes comprehensive documentation covering edge cases such as read-only root filesystems, environment variable handling, and performance considerations.
