Refactor createWorker() API

Use postfix as int instead string. This make the following patch easier
to read.

cli/commands.go

@@ -148,7 +148,7 @@ func CreateCluster(c *cli.Context) error {
 				env,
 				c.String("name"),
 				strings.Split(c.String("volume"), ","),
-				strconv.Itoa(i),
+				i,
 				c.String("port"),
 			)
 			if err != nil {

cli/container.go

@@ -190,14 +190,14 @@ func createServer(verbose bool, image string, port string, args []string, env []
 }
 
 // createWorker creates/starts a k3s agent node that connects to the server
-func createWorker(verbose bool, image string, args []string, env []string, name string, volumes []string, postfix string, serverPort string) (string, error) {
+func createWorker(verbose bool, image string, args []string, env []string, name string, volumes []string, postfix int, serverPort string) (string, error) {
 	containerLabels := make(map[string]string)
 	containerLabels["app"] = "k3d"
 	containerLabels["component"] = "worker"
 	containerLabels["created"] = time.Now().Format("2006-01-02 15:04:05")
 	containerLabels["cluster"] = name
 
-	containerName := fmt.Sprintf("k3d-%s-worker-%s", name, postfix)
+	containerName := fmt.Sprintf("k3d-%s-worker-%d", name, postfix)
 
 	env = append(env, fmt.Sprintf("K3S_URL=https://k3d-%s-server:%s", name, serverPort))