The --allow-nondistributable-artifacts daemon option specifies
registries to which foreign layers should be pushed. (By default,
foreign layers are not pushed to registries.)
Additionally, to make this option effective, foreign layers are now
pulled from the registry if possible, falling back to the URLs in the
image manifest otherwise.
This option is useful when pushing images containing foreign layers to a
registry on an air-gapped network so hosts on that network can pull the
images without connecting to another server.
Signed-off-by: Noah Treuhaft <noah.treuhaft@docker.com>