Each image consists of a series of layers which are stacked, one on top of the another. In fact, every layer is an intermediate image. By using the union filesystem, Docker combines all these layers into a single image entity. The union filesystem allows transparent overlaying files and directories of separate filesystems, giving a single, consistent filesystem as a result, as you can see the following diagram:
Contents and structure of directories which have the same path within these separate filesystems will be seen together in a single merged directory, within the new, virtual-like filesystem. In other words, the filesystem structure ...
