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>
c96373f6d5