Add support for getting archive of container rootfs changes

[jlhawn]

The docker export command and corresponding API endpoint allow the client to get an archive of the container's root filesystem. This is useful for squashing an entire set of image layers into one, but there's currently no way to get just the file system diff for a container.

This could be achieved by adding an optional parameter to the container export API endpoint and an optional flag to the docker export command: diff=1 This would instruct the runtime to only return the layer diff for the container rather than the entire filesystem.

Why is this useful?

It would allow for additional functionality in tools which are used to build images like the docker build command. Layer archives could be collected by the client, checksums computed for each layer archive, and for images to be built using a new content-addressable format.

[stevvooe]

#11104 was a PR for this function by hacking the "docker save" output. It would be somewhat nice to have access to individual layers by id. Not sure how this would integrate but the concept is sound.

[LK4D4]

@tonistiigi @stevvooe Do you think this is still actual?

[stevvooe]

@LK4D4 I think there is still a general need to be able to access layers from an API on the daemon. There is a solution to this problem that involves integrating a registry and providing other access paths to registry data, such as uncompressed hash identifiers.