Docker is an open source platform that packages an application and all its dependencies into a container, then runs it identically on a developer laptop, a server or in the cloud. It was introduced in 2013 by Solomon Hykes, at the company dotCloud.
It relies on three notions. The Dockerfile describes, step by step, how to build the application's environment. The image is the result of that build, a frozen template that can be shared through a registry such as Docker Hub. The container is a running image, isolated from the rest of the machine.
Docker brought containers to every developer: it simplifies setting up a project, guarantees consistency between environments and serves as the basis for CI/CD pipelines. To run many containers in production, teams rely on an orchestrator such as Kubernetes. Docker Compose, for its part, is used to start several containers together locally.
Why it matters when hiring
Docker is no longer a differentiating skill for a back-end developer: it is a prerequisite. For a DevOps or Platform role, go further in the interview: image size optimization, security (vulnerability scanning, non-root users), secrets management. The trap: taking "Docker" on a resume as experience running infrastructure in production.