What is Docker?

Docker is a software platform that automates the deployment of applications inside isolated user-space instances called containers. It uses operating-system-level virtualisation, primarily via Linux kernel features such as cgroups and namespaces, to run multiple isolated workloads on a single host without the overhead of a hypervisor or full virtual machines. Docker popularised the container format and tooling, providing a standardised way to build, ship, and run software.

At the core of Docker is the Docker Engine, a client-server application that includes a daemon (dockerd), a REST API, and a command-line interface (CLI). The daemon manages images, containers, networks, and storage volumes. Images are built from Dockerfiles, which specify a base image, application code, dependencies, and runtime commands. Images are stored in registries, the default being Docker Hub, and can be versioned and shared. Each container is an instantiation of an image, with its own filesystem, network stack, and process tree.

Docker's introduction in 2013 radically simplified the developer workflow for creating reproducible environments and deploying microservices. It sits between the host operating system and the application, abstracting infrastructure dependencies. The Open Container Initiative (OCI) later standardised the image format and runtime, ensuring interoperability between Docker and other container engines such as Podman and containerd. Despite competition, Docker remains the de facto reference implementation for container development, though in production environments it is often replaced by orchestration platforms like Kubernetes that use Docker as a runtime or its underlying components directly.