What is a Docker volume?

As we learned in Chapter 3, Creating Docker Images, Docker uses a special filesystem called a Union File System. This is the key to Docker's layered image model and allows for many of the features that make using Docker so desirable. However, the one thing that the Union File System does not provide for is the persistent storage of data. If you recall, the layers of a Docker image are read-only. When you run a container from a Docker image, the Docker daemon creates a new read-write layer that holds all of the live data that represents your container. When your container makes changes to its filesystem, those changes go into that read-write layer. As such, when your container goes away, taking the read-write layer goes with it, and any and all changes the container made to data within that layer are deleted and gone forever. That equals non-persistent...