In the previous chapter, we learned what a container is. The isolated process or processes running inside a container will have their own root filesystem among other namespaces. The container adds a thin layer on top of image layers and every change made during the execution of its processes will be stored only on this layer. In order to manage these changes, the Docker storage driver will use stackable layers and copy-on-write (sometimes referenced as CoW).

When a process inside a container needs to modify a file, the Docker daemon storage filesystem mechanism will make a copy of that file from the underlying layers to the top one. These are only available for container usage. The same happens when a new file is created; it will only be written to the top container storage layer. All the other processes running on other containers will manage their own version of the file. In fact, this will be the original file from the other layers if no changes...