fix: clean up local build images immediately after an image is released

In some cases, we might hold onto intermediate images until the next deploy. While these images are generally part of newer images - and are therefore cleaned up when the child images are no longer in use - they cruft up the 'docker image ls' output, causing some folks to believe Dokku is not cleaning up images.

Refs #6272

plugins/builder/src/triggers/triggers.go

@@ -50,6 +50,10 @@ func main() {
 	case "post-delete":
 		appName := flag.Arg(0)
 		err = builder.TriggerPostDelete(appName)
+	case "post-release-builder":
+		builderType := flag.Arg(0)
+		appName := flag.Arg(1)
+		err = builder.TriggerPostReleaseBuilder(builderType, appName)
 	case "report":
 		appName := flag.Arg(0)
 		err = builder.ReportSingleApp(appName, "", "")