Correct build-time directory creation with user namespaced daemon

This fixes errors in ownership on directory creation during build that
can cause inaccessible files depending on the paths in the Dockerfile
and non-existing directories in the starting image.

Add tests for the mkdir variants in pkg/idtools

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

idtools/idtools_windows.go

@@ -0,0 +1,18 @@
+// +build windows
+
+package idtools
+
+import (
+	"os"
+
+	"github.com/docker/docker/pkg/system"
+)
+
+// Platforms such as Windows do not support the UID/GID concept. So make this
+// just a wrapper around system.MkdirAll.
+func mkdirAs(path string, mode os.FileMode, ownerUID, ownerGID int, mkAll, chownExisting bool) error {
+	if err := system.MkdirAll(path, mode); err != nil && !os.IsExist(err) {
+		return err
+	}
+	return nil
+}