The writable container layer

As we have discussed, a container image is made of a stack of immutable or read-only layers. When the Docker engine creates a container from such an image, it adds a writable container layer on top of this stack of immutable layers. Our stack now looks as follows:

The container layer is marked as read/write. Another advantage of the immutability of image layers is that they can be shared among many containers created from this image. All that is needed is a thin, writable container layer for each container:

This technique, of course, results in a tremendous reduction of resources that are consumed. Furthermore, this helps to decrease the loading time of a container since only a thin container layer has to be created once the image layers have been loaded into memory, which only happens for the first container.