feat(register): add label format validation

cmd/register_test.go

@@ -0,0 +1,10 @@
+package cmd
+
+import "testing"
+
+func TestValidateLabels(t *testing.T) {
+	labels := []string{"ubuntu-latest:docker://node:16-buster"}
+	if err := validateLabels(labels); err != nil {
+		t.Errorf("validateLabels() error = %v", err)
+	}
+}