implementing the support for the registry mirror

DOCS.md

@@ -8,6 +8,7 @@ The following parameters are used to configure this plugin:
 * `repo` - repository name for the image
 * `tag` - repository tag for the image
 * `insecure` - enable insecure communication to this registry
+* `mirror` - use a mirror registry instead of pulling images directly from the central Hub
 * `storage_driver` - use `aufs`, `devicemapper`, `btrfs` or `overlay` driver
 * `archive` - save and restore image layers to/from a tarred archive
     * `file` - absolute or relative path to archive file

main.go

@@ -20,6 +20,7 @@ type Archive struct {
 type Docker struct {
 	Storage  string   `json:"storage_driver"`
 	Registry string   `json:"registry"`
+	Mirror   string   `json:"mirror"`
 	Insecure bool     `json:"insecure"`
 	Username string   `json:"username"`
 	Password string   `json:"password"`
@@ -82,6 +83,9 @@ func main() {
 		if vargs.Insecure && len(vargs.Registry) != 0 {
 			args = append(args, "--insecure-registry", vargs.Registry)
 		}
+		if len(vargs.Mirror) != 0 {
+			args = append(args, "--registry-mirror", vargs.Mirror)
+		}
 
 		for _, value := range vargs.Dns {
 			args = append(args, "--dns", value)