Copy-On-Write
Before Docker, LXC would create a full copy of FileSystem when creating a container. This would be slow and take up a lot of space. When Docker creates a container, it adds a new, thin, writable layer on top of the underlying stack of image layers. This layer is often called the “container layer”.
All changes made to the running container - such as writing new files, modifying existing files, and deleting files - are written to this thin writable container layer.