Packaging an application together with everything it needs to run — runtime, libraries, configuration — into an image that behaves identically on a laptop, in testing, and in production.
Containers end the class of problems that start with it works on my machine. They also make environments reproducible, which is the precondition for reliable automated deployment and for onboarding a new developer in an hour.
We containerise every project, use the same image across environments with configuration injected at runtime, and keep images small so builds and deployments stay fast.