volume Create: fix incorrect file permissions (staticcheck)

```
14:01:54 volume/local/local.go:175:80: SA9002: file mode '600' evaluates to 01130; did you mean '0600'? (staticcheck)
14:01:54 		if err = ioutil.WriteFile(filepath.Join(filepath.Dir(path), "opts.json"), b, 600); err != nil {
```

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

volume/local/local.go

@@ -172,7 +172,7 @@ func (r *Root) Create(name string, opts map[string]string) (volume.Volume, error
 		if err != nil {
 			return nil, err
 		}
-		if err = ioutil.WriteFile(filepath.Join(filepath.Dir(path), "opts.json"), b, 600); err != nil {
+		if err = ioutil.WriteFile(filepath.Join(filepath.Dir(path), "opts.json"), b, 0600); err != nil {
 			return nil, errdefs.System(errors.Wrap(err, "error while persisting volume options"))
 		}
 	}