fix(cmd): add flag first-win

cmd/root.go

@@ -42,6 +42,7 @@ dcmerge docker-compose.yml https://git.example.local/user/repo/docker-compose.ym
 		RunE:    run,
 		Version: version,
 	}
+	rootCmd.Flags().BoolP("first-win", "f", false, "Add only missing attributes")
 	rootCmd.Flags().BoolP("last-win", "l", false, "Overwrite existing attributes")
 	rootCmd.Flags().StringP("output-file", "o", "", "Write instead on stdout into a file")
 	rootCmd.AddCommand(completionCmd)
@@ -50,6 +51,11 @@ dcmerge docker-compose.yml https://git.example.local/user/repo/docker-compose.ym
 }
 
 func run(cmd *cobra.Command, args []string) error {
+	mergeFirstWin, err := cmd.Flags().GetBool("first-win")
+	if err != nil {
+		return fmt.Errorf("Failed to parse flag last-win: %s", err)
+	}
+
 	mergeLastWin, err := cmd.Flags().GetBool("last-win")
 	if err != nil {
 		return fmt.Errorf("Failed to parse flag last-win: %s", err)
@@ -69,12 +75,15 @@ func run(cmd *cobra.Command, args []string) error {
 
 	for _, config := range dockerComposeConfigs {
 		switch {
-		case mergeLastWin:
+		case mergeFirstWin && mergeLastWin:
+			return fmt.Errorf("Neither --first-win or --last-win can be specified - not booth.")
+		case mergeFirstWin && !mergeLastWin:
+			dockerComposeConfig.MergeFirstWin(config)
+		case !mergeFirstWin && mergeLastWin:
 			dockerComposeConfig.MergeLastWin(config)
 		default:
 			dockerComposeConfig.Merge(config)
 		}
-
 	}
 
 	switch {