Well, this might not even be worth wasting precious characters of this blog article, but when each byte counts, each layer counts. The metadata of an image layer contains information such as which command was used, operating system, history and changes in environment variables. All saved as JSON data. (The location on the file system depends on your Docker setup, we are using overlay2 as storage, which stores all of the metadata in /var/lib/docker/image/overlay2/imagedb/content/sha256/* with our setup.)