What is an image layer in Docker?

A layer in Docker is a separate layer of the filesystem that represents changes relative to the previous layer. Each layer contains only deltas (changes), such as added, modified, or deleted files.

Features:

• Layers are immutable after creation.
• Layers are stacked on top of each other, forming the final image.
• Using layers allows for efficient caching and reuse of parts of images during build.

For example, if you have a base layer with an OS, and then a layer with an installed application, Docker stores them separately. When the application changes, a new layer is created, while the base remains unchanged.

This reduces storage volume and speeds up image loading, as common layers can be reused.