Fix Gradle build step in Jenkinsfile

Jenkinsfile

@@ -3,7 +3,10 @@ pipeline {
     stages {
         stage('Gradle') {
             steps {
-                sh 'bash gradlew build'
+                sh '''
+                    chmod +x gradlew
+                    ./gradlew build
+                '''
             }
         }
         stage('Archive') {