fix: make release not crash on missing images

cli/recipe/release.go

@@ -287,6 +287,9 @@ func getImageVersions(recipe recipe.Recipe) map[string]string {
 
 	var services = make(map[string]string)
 	for _, service := range recipe.Config.Services {
+		if (service.Image == "") {
+			continue
+		}
 		srv := strings.Split(service.Image, ":")
 		services[srv[0]] = srv[1]
 	}