Docker

Docker is the tooling that made containers ordinary: a format for describing an image, a way to build one, and a runtime to start it. When somebody says an application is containerised, they usually mean there is a Dockerfile describing how to build it.

For a project of modest size, Docker with a compose file is often the whole of the infrastructure story: a few services declared in one file, started together, restarted by the host if they fail. That is enough for a great many production systems, and it is a great deal less to operate than an orchestrator.

The habit worth forming early is treating the image as immutable and the configuration as external. Secrets and environment-specific values are supplied at run time, so the same image can be promoted from staging to production without a rebuild.