Fix dockerTag property resolution

Author: sjvanrossum

runners/google-cloud-dataflow-java/build.gradle

@@ -316,12 +316,8 @@ tasks.register('buildAndPushDistrolessContainerImage', Task.class) {
   if (!javaVer in ["java17", "java21"]) {
       throw new GradleException("testJavaVersion must be one of ${allowed}, got: ${javaVer}")
     }
-    if (!project.hasProperty('dockerTag')) {
-      throw new GradleException("dockerTag is missing but required")
-    }
     def repository = "us.gcr.io/apache-beam-testing/${System.getenv('USER')}"
-    def tag = project.findProperty('dockerTag')
-    def imageURL = "${repository}/beam_${javaVer}_sdk_distroless:${tag}"
+    def imageURL = "${repository}/beam_${javaVer}_sdk_distroless:${dockerTag}"
     exec {
       executable 'docker'
       workingDir rootDir