Container

A container packages an application together with everything it needs to run — libraries, runtime, configuration — so that it behaves identically wherever it is started. It is the practical answer to "it works on my machine".

Unlike a virtual machine, a container shares the host's operating system kernel rather than carrying its own. That makes it small and quick to start, which is what allows a single server to run many of them and a deployment to be measured in seconds.

The operational benefit is that the unit you test is the unit you deploy. A container that passed in staging is byte-for-byte the thing running in production, which removes a whole category of failure caused by environments drifting apart.