About
A layer is just the equivalent of a tarball
A layer is created at built time via each Dockerfile command. ie each command in the Dockerfile becomes a new layer in the image.
Once a layer changes, all downstream layers have to be recreated as well
Command responsible for the layer creation
Using the docker image history command 1), you can see the docker command that was used to create each layer within an image.
Example:
docker image history --no-trunc image-name