newTransport(): remove deprecated DualStack option

The `DualStack` option was deprecated in Go 1.12, and is now enabled by default
(through commit github.com/golang/go@efc185029bf770894defe63cec2c72a4c84b2ee9).

> The Dialer.DualStack field is now meaningless and documented as deprecated.
>
> To disable fallback, set FallbackDelay to a negative value.

The default `FallbackDelay` is 300ms; to make this more explicit, this patch
sets `FallbackDelay` to the default value.

Note that Docker Hub currently does not support IPv6 (DNS for registry-1.docker.io
has no AAAA records, so we should not hit the 300ms delay).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/server/image_pull.go

@@ -425,9 +425,9 @@ func newTransport() *http.Transport {
 	return &http.Transport{
 		Proxy: http.ProxyFromEnvironment,
 		DialContext: (&net.Dialer{
-			Timeout:   30 * time.Second,
-			KeepAlive: 30 * time.Second,
-			DualStack: true,
+			Timeout:       30 * time.Second,
+			KeepAlive:     30 * time.Second,
+			FallbackDelay: 300 * time.Millisecond,
 		}).DialContext,
 		MaxIdleConns:          10,
 		IdleConnTimeout:       30 * time.Second,