Remove feature flag "windows-dns-proxy"

[robmry]

- What I did

• fix Remove windows-dns-proxy feature flag #47732

Added in 26.1.0, commit 6c68be2
Default changed to true in 27.0.0, commit 33f9a53

- How I did it

No sign of problems so, removed.

- How to verify it

It doesn't do anything now.

- Description for the changelog

- Removed feature flag `windows-dns-proxy`, which was introduced in release 26.1.0 to enable forwarding
  to external DNS resolvers from Windows containers, to make `nslookup` work. It was enabled by default
  in release 27.0.0.

[thaJeztah]

Not sure how likely it is, but the only thing we could do is to (in the Windows variant of this) either return an error, or to log a Error / Warning to surface that the user is using an option that's no longer taken into account; given that the default is "true", that should probably only be relevant if they have it configured and set to false;

moby/daemon/config/config.go

Lines 687 to 693 in 43bbacb

	if config.CorsHeaders != "" {
	// TODO(thaJeztah): option is used to produce error when used; remove in next release
	return errors.New(`DEPRECATED: The "api-cors-header" config parameter and the dockerd "--api-cors-header" option have been removed; use a reverse proxy if you need CORS headers`)
	}
	// validate platform-specific settings
	return config.ValidatePlatformConfig()

Alternative to logging, we could add it to the Warnings array on the /info endpoint, which shows up in the output of docker info;

moby/daemon/info.go

Lines 46 to 50 in 43bbacb

	func (daemon *Daemon) SystemInfo(ctx context.Context) (*system.Info, error) {
	defer metrics.StartTimer(hostInfoFunctions.WithValues("system_info"))()
	sysInfo := daemon.RawSysInfo()
	cfg := daemon.config()

moby/daemon/info.go

Lines 153 to 165 in 43bbacb

	func (daemon *Daemon) fillDriverInfo(v *system.Info) {
	v.Driver = daemon.imageService.StorageDriver()
	v.DriverStatus = daemon.imageService.LayerStoreStatus()
	const warnMsg = `
	WARNING: The %s storage-driver is deprecated, and will be removed in a future release.
	Refer to the documentation for more information: https://docs.docker.com/go/storage-driver/`
	switch v.Driver {
	case "overlay":
	v.Warnings = append(v.Warnings, fmt.Sprintf(warnMsg, v.Driver))
	}

[robmry]

Ok, sure - I've added a docker info warning (and I'll mark this as ready for review!), how does this look? ...

PS C:\ProgramData\docker> docker info
Client:
 Version:    27.3.0-rc.1
[...]
 Insecure Registries:
  ::1/128
  127.0.0.0/8
 Live Restore Enabled: false


WARNING: Feature flag "windows-dns-proxy" has been removed, forwarding to external DNS resolvers is enabled.

[thaJeztah]

LGTM

[thaJeztah]

Thanks! Yes, I was thinking we'd need it only if someone has false set, but I guess this is good as well to tell them "remove this from your config"

We can remove this in v29.0 or so (maybe earlier, it's just a warning)