A named tool for running things in isolated containers. A named tool rather than a concept.
Middle Dutch docke, a dock or basin where ships load and unload; a docker is one who works that loading, and the software tool borrows the same image for the containers it packs and ships.
A named tool for running things sealed inside their own container, borrowing its whole metaphor from the work of loading a ship. What it carries travels intact, regardless of the dock it lands at.