Each image consists of a series of layers that are stacked on one another. By using the union filesystem, Docker combines all these layers into a single image entity. The union filesystem allows transparent overlaying of files and directories of separate filesystems, giving a single, consistent filesystem as a result, as you can see in the following diagram:

The contents and structure of directories that 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 of the top layer will merge with the structure of the layer beneath. Files and directories that have the same path as in the previous layer will cover those beneath. Removing the upper layer will again reveal and expose the previous directory content. As we have mentioned earlier, layers are placed in a stack on top of one another. To maintain the order of layers, Docker utilizes the concept of layer...