gradle plugin: Fix tag task dependency with proper format string (apache-spark-on-k8s#453)

Author: mccheah

spark-docker-image-generator/src/main/java/org/apache/spark/deploy/kubernetes/docker/gradle/SparkDockerPlugin.java

@@ -163,7 +163,7 @@ private void setupDockerTasks(
                   task ->
                       task.commandLine(
                           "docker", "push", String.format("%s:%s", resolvedImageName, tag))
-                          .dependsOn("sparkDockerTag%s")))
+                          .dependsOn(String.format("sparkDockerTag%s", tag))))
           .collect(Collectors.toList());
       if (!pushTasks.isEmpty()) {
         pushAllTask.dependsOn(pushTasks);