How are builds cached in Docker?

In Docker, builds are cached primarily because each instruction in a Dockerfile creates a separate layer. This layer caching mechanism is fundamental to how Docker optimizes the build process. When you build an image, Docker executes each command in the Dockerfile sequentially, creating a new layer for each command. These layers are stored in the local Docker cache, and if a future build uses the same command, Docker can reuse the cached layer instead of re-executing the command.

Consequently, when you modify a Dockerfile, only the layers from the modified instruction onward need to be rebuilt. This significantly speeds up the build process, especially when working with large images or complex configurations, as unchanged layers can be retrieved from the cache rather than being rebuilt from scratch. This mechanism enhances efficiency by reducing the time and resources consumed during the build process.

The other options do not accurately capture how Docker handles caching during the build process. Docker volumes are used for persistent storage but are unrelated to build caching. Saving images to a repository allows sharing and distribution of images but does not relate directly to the caching of builds. Configuration file backups, while important for other reasons, do not pertain to the caching system used in Docker builds.