Fix WCOW COPY --from failure in multistage builds on Windows

Fixes: moby#5193

Signed-off-by: Billy Owire <[email protected]>

cache/contenthash/checksum.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"crypto/sha256"
+	"github.com/Microsoft/go-winio"
 	"io"
 	"os"
 	"path"
@@ -1034,7 +1035,7 @@ func (cc *cacheContext) scanPath(ctx context.Context, m *mount, p string, follow
 		scanPath = resolvedPath
 	}
 
-	err = filepath.Walk(scanPath, func(itemPath string, fi os.FileInfo, err error) error {
+	walkFn := func(itemPath string, fi os.FileInfo, err error) error {
 		if scanCounterEnable {
 			scanCounter.Add(1)
 		}
@@ -1073,7 +1074,13 @@ func (cc *cacheContext) scanPath(ctx context.Context, m *mount, p string, follow
 			txn.Insert(k, cr)
 		}
 		return nil
+	}
+
+	privileges := []string{winio.SeBackupPrivilege}
+	err = winio.RunWithPrivileges(privileges, func() error {
+		return filepath.Walk(scanPath, walkFn)
 	})
+
 	if err != nil {
 		return err
 	}