gradle: export runnable jar

app/build.gradle

@@ -36,6 +36,13 @@ dependencies {
 application {
     // Define the main class for the application.
     mainClass = "chess.App"
+    applicationName = "3DChess"
+}
+
+jar {
+    manifest {
+        attributes 'Main-Class': application.mainClass
+    }
 }
 
 tasks.named('test') {