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)

opts/opts.go

@@ -3,6 +3,7 @@ package opts
 import (
 	"fmt"
 	"net"
+	"net/url"
 	"os"
 	"path/filepath"
 	"regexp"
@@ -33,6 +34,10 @@ func IPVar(value *net.IP, names []string, defaultValue, usage string) {
 	flag.Var(NewIpOpt(value, defaultValue), names, usage)
 }
 
+func MirrorListVar(values *[]string, names []string, usage string) {
+	flag.Var(newListOptsRef(values, ValidateMirror), names, usage)
+}
+
 // ListOpts type
 type ListOpts struct {
 	values    *[]string
@@ -190,3 +195,21 @@ func validateDomain(val string) (string, error) {
 	}
 	return "", fmt.Errorf("%s is not a valid domain", val)
 }
+
+// Validates an HTTP(S) registry mirror
+func ValidateMirror(val string) (string, error) {
+	uri, err := url.Parse(val)
+	if err != nil {
+		return "", fmt.Errorf("%s is not a valid URI", val)
+	}
+
+	if uri.Scheme != "http" && uri.Scheme != "https" {
+		return "", fmt.Errorf("Unsupported scheme %s", uri.Scheme)
+	}
+
+	if uri.Path != "" || uri.RawQuery != "" || uri.Fragment != "" {
+		return "", fmt.Errorf("Unsupported path/query/fragment at end of the URI")
+	}
+
+	return fmt.Sprintf("%s://%s/v1/", uri.Scheme, uri.Host), nil
+}