Fix: Typo in script

Jenkinsfile

@@ -9,11 +9,10 @@ String jenkinsVersion
 def publishDockerImage(String jenkinsVersion) {
     docker.withRegistry("https://registry.hub.docker.com", 'teeckebot-docker-credentials') {
         script (
-            sh """#!/bin/bash
-            for tag in `cat build-tools.list`; do
+            sh '''for tag in $(cat build-tools.list); do
                 docker push teecke/docker-flutter-builder:${tag}
             done
-            """
+            '''
         )
     }
 }