LCOW: Auto-select OS

Signed-off-by: John Howard <jhoward@microsoft.com>

Addresses https://github.com/moby/moby/pull/35089#issuecomment-367802698.
This change enables the daemon to automatically select an image under LCOW
that can be used if the API doesn't specify an explicit platform.

For example:

FROM supertest2014/nyan
ADD Dockerfile /

And docker build . will download the linux image (not a multi-manifest image)

And similarly docker pull ubuntu will match linux/amd64
Upstream-commit: 35193c0e7dc301e1d2f6ea96e0ce34ffd2d4b88d
Component: engine

components/engine/distribution/pull.go

@@ -3,7 +3,6 @@ package distribution // import "github.com/docker/docker/distribution"
 import (
 	"context"
 	"fmt"
-	"runtime"
 
 	"github.com/docker/distribution/reference"
 	"github.com/docker/docker/api"
@@ -115,11 +114,6 @@ func Pull(ctx context.Context, ref reference.Named, imagePullConfig *ImagePullCo
 			continue
 		}
 
-		// Make sure we default the OS if it hasn't been supplied
-		if imagePullConfig.OS == "" {
-			imagePullConfig.OS = runtime.GOOS
-		}
-
 		if err := puller.Pull(ctx, ref, imagePullConfig.OS); err != nil {
 			// Was this pull cancelled? If so, don't try to fall
 			// back.