Support `nerdctl save [CONTAINER IMAGE ID]`

[containerd]

Support `nerdctl save [CONTAINER IMAGE ID]`

[AkihiroSuda]

OCI Archive with manifest.json is compatible with Docker Image Spec

[soharaki]

@AkihiroSuda
Hi, when I export a tarball directly using client.Export will output the OCI Image Spec tar instead of a Docker Image Spec v1.2 tar.

• Any client.Export options or alt-implementations which can export the Docker Image Spec v1.2?
• Should I explicitly explain that it is the OCI Image Spec tar in the help?

Any advices. Thank you.

• refs:

func saveContainer(images []string, out io.Writer, saveOpts []archive.ExportOpt, clicontext *cli.Context) error {
	client, ctx, cancel, err := newClient(clicontext)
	if err != nil {
		return err
	}
	defer cancel()

	// Set the default platform
	saveOpts = append(saveOpts, archive.WithPlatform(platforms.Default()))

	is := client.ImageService()
	for _, img := range images {
		saveOpts = append(saveOpts, archive.WithImage(is, img))
	}

	return client.Export(ctx, out, saveOpts...)
}