Layers
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 ...
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