Address comments from Daniel and solve the lint error.

Author: lipingxue

cli/compose/convert/volume.go

@@ -61,16 +61,19 @@ func convertVolumeToMount(
 		return result, errors.Errorf("undefined volume %q", volume.Source)
 	}
 
-	if stackVolume.External.External && stackVolume.External.Name != "" {
-		return result, errors.Errorf("cannot specify external volume name %q", stackVolume.External.Name)
-	}
 	result.Source = namespace.Scope(volume.Source)
 	result.VolumeOptions = &mount.VolumeOptions{}
 
 	if volume.Volume != nil {
 		result.VolumeOptions.NoCopy = volume.Volume.NoCopy
 	}
 
+	// External named volumes
+	if stackVolume.External.External {
+		result.Source = stackVolume.External.Name
+		return result, nil
+	}
+
 	if stackVolume.Name != "" {
 		result.Source = stackVolume.Name
 	}

cli/compose/types/types.go

@@ -305,7 +305,7 @@ type IPAMPool struct {
 
 // VolumeConfig for a volume
 type VolumeConfig struct {
-	Name	   string
+	Name       string
 	Driver     string
 	DriverOpts map[string]string `mapstructure:"driver_opts"`
 	External   External