Windows: Fix long path handling for docker build

Signed-off-by: Stefan J. Wernli <swernli@microsoft.com>
Upstream-commit: 9b648dfac6453de5944ee4bb749115d85a253a05
Component: engine

components/engine/pkg/directory/directory_windows.go

@@ -5,10 +5,18 @@ package directory
 import (
 	"os"
 	"path/filepath"
+	"strings"
+
+	"github.com/docker/docker/pkg/longpath"
 )
 
 // Size walks a directory tree and returns its total size in bytes.
 func Size(dir string) (size int64, err error) {
+	fixedPath, err := filepath.Abs(dir)
+	if err != nil {
+		return
+	}
+	fixedPath = longpath.AddPrefix(fixedPath)
 	err = filepath.Walk(dir, func(d string, fileInfo os.FileInfo, e error) error {
 		// Ignore directory sizes
 		if fileInfo == nil {