The document discusses building custom tools using the Docker API and SDK. It begins with an overview of Docker and its architecture. It then covers connecting to remote Docker hosts via TCP and configuring the Docker daemon. The Docker API and SDKs for various languages are introduced. The problem of managing many containers across multiple servers is presented as motivation for building a custom tool. DockerDoodle is presented as a web application built using the Docker API to monitor and manage containers on remote Docker hosts. Its architecture connects to Docker daemons via TCP and a web socket interface.