Codetainer is a browser-based sandbox for running Docker containers. It allows users to "try 'X' in your browser" for any X by running Docker containers in an isolated and programmable manner directly in the browser. Codetainer uses Docker APIs to launch and manage lightweight containers via a Go-based API server. Users can create and register Docker images, launch "codetainers" from those images, and interact with the codetainers through the browser via websockets, viewing terminals and sending keystrokes. Codetainer aims to provide a secure and flexible environment for use cases like tutorials, training, and remote management while addressing challenges around container introspection and security.