You should remember that a Docker image is made up of a series of read-only layers built upon one another, and that when you run a container from a Docker image, it creates a new read-write layer that you can think of as being on top of the read-only layers. All the changes to the container are applied to the read-write layer. If you make a change to a file found in one of the read-only layers, a copy of that file is made and added to the read-write layer. Then, all the changes are applied to the copy. The copy hides the version found in the read-only layer so, from the point of view of the running container, there is only one version of the file, and it is the one that has been changed. This is roughly how the Unified File System works.

This is actually a great thing. However, it presents a challenge, this being that when the running container exits and...