Add daemon flag to specify public registry mirrors

Adds support for a --registry-mirror=scheme://<host>[:port]
daemon flag. The flag may be present multiple times. If
provided, mirrors are prepended to the list of endpoints used
for image pull. Note that only mirrors of the public
index.docker.io registry are supported, and image/tag resolution
is still performed via the official index.

Docker-DCO-1.1-Signed-off-by: Tim Smith <timbot@google.com> (github: timbot)

daemon/config.go

@@ -23,6 +23,7 @@ type Config struct {
 	AutoRestart                 bool
 	Dns                         []string
 	DnsSearch                   []string
+	Mirrors                     []string
 	EnableIptables              bool
 	EnableIpForward             bool
 	DefaultIp                   net.IP
@@ -60,6 +61,7 @@ func (config *Config) InstallFlags() {
 	// FIXME: why the inconsistency between "hosts" and "sockets"?
 	opts.IPListVar(&config.Dns, []string{"#dns", "-dns"}, "Force Docker to use specific DNS servers")
 	opts.DnsSearchListVar(&config.DnsSearch, []string{"-dns-search"}, "Force Docker to use specific DNS search domains")
+	opts.MirrorListVar(&config.Mirrors, []string{"-registry-mirror"}, "Specify a preferred Docker registry mirror")
 }
 
 func GetDefaultNetworkMtu() int {

daemon/daemon.go

@@ -791,7 +791,7 @@ func NewDaemonFromDirectory(config *Config, eng *engine.Engine) (*Daemon, error)
 		return nil, err
 	}
 	log.Debugf("Creating repository list")
-	repositories, err := graph.NewTagStore(path.Join(config.Root, "repositories-"+driver.String()), g)
+	repositories, err := graph.NewTagStore(path.Join(config.Root, "repositories-"+driver.String()), g, config.Mirrors)
 	if err != nil {
 		return nil, fmt.Errorf("Couldn't create Tag store: %s", err)
 	}