Introduce the sync command

The skopeo sync command can sync images between a SOURCE and a
destination.

The purpose of this command is to assist with the mirroring of
container images from different docker registries to a single
docker registry.

Right now the following source/destination locations are implemented:

  * docker -> docker
  * docker-> dir
  * dir -> docker

The dir location is supported to handle the use case
of air-gapped environments.
In this context users can perform an initial sync on a trusted machine
connected to the internet; that would be a `docker` -> `dir` sync.
The target directory can be copied to a removable drive that can then be
plugged into a node of the air-gapped environment. From there a
`dir` -> `docker` sync will import all the images into the registry serving
the air-gapped environment.

Notes when specifying the `--scoped` option:

The image namespace is changed during the  `docker` to `docker` or `dir` copy.
The FQDN of the registry hosting the image will be added as new root namespace
of the image. For example, the image `registry.example.com/busybox:latest`
will be copied to
`registry.local.lan/registry.example.com/busybox:latest`.

The image namespace is not changed when doing a
`dir:` -> `docker` sync operation.

The alteration of the image namespace is used to nicely scope images
coming from different registries (the Docker Hub, quay.io, gcr,
other registries). That allows all of them to be hosted on the same
registry without incurring in clashes and making their origin explicit.

Signed-off-by: Flavio Castelli <[email protected]>
Co-authored-by: Marco Vedovati <[email protected]>

cmd/skopeo/main.go

@@ -98,6 +98,7 @@ func createApp() (*cli.App, *globalOptions) {
 		layersCmd(&opts),
 		deleteCmd(&opts),
 		manifestDigestCmd(),
+		syncCmd(&opts),
 		standaloneSignCmd(),
 		standaloneVerifyCmd(),
 		untrustedSignatureDumpCmd(),