How to explain the principle of layered Docker containers and why it is needed?

Docker containers are built from layers, each layer representing changes relative to the previous one. This is similar to a stack of layers, where each layer adds or modifies files.

The layering principle is needed for:

• Efficient use of disk space: shared layers between containers are stored only once.
• Fast creation and updating of images: when a change occurs, only the top layer is modified, the others remain unchanged.
• Caching: Docker reuses layers if they haven't changed.

For example, if you have an image with a basic OS and you add an application — the basic OS will be in one layer, and the application in another. If you create a new image with a different application, the basic OS will not be downloaded again.

This speeds up the building and running of containers, and also saves disk space.