fix build after merge of outdated PR

this broke after e554ab558985b686c9c3427275a5e016aa1cdb76,
was merged but the PR was outdated.

```
[2019-08-28T00:27:12.031Z] builder/dockerfile/copy.go:566:29: too many arguments in call to system.MkdirAll
[2019-08-28T00:27:12.031Z] 	have (string, number, string)
[2019-08-28T00:27:12.031Z] 	want (string, os.FileMode)
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

builder/dockerfile/copy.go

@@ -563,7 +563,7 @@ func copyFile(archiver Archiver, source, dest *copyEndpoint, identity *idtools.I
 			// are of the form \\?\Volume{<GUID>}\<path>. An example would be:
 			// \\?\Volume{dae8d3ac-b9a1-11e9-88eb-e8554b2ba1db}\bin\busybox.exe
 
-			if err := system.MkdirAll(filepath.Dir(dest.path), 0755, ""); err != nil {
+			if err := system.MkdirAll(filepath.Dir(dest.path), 0755); err != nil {
 				return err
 			}
 		} else {