[Gradle] Clean up DataflowRunner's Java distroless container image build tasks

[sjvanrossum]

Parts of this build file appear to be duplicated and/or outdated.

The container image build script in this build file:

tasks.register('buildAndPushDistrolessContainerImage', Task.class) {
  // Only Java 17 and 21 are supported.
  // See https://github.com/GoogleContainerTools/distroless/tree/main/java#image-contents.
  def allowed = ["java17", "java21"]
  doLast {
    def javaVer = project.findProperty('testJavaVersion')
    if (!allowed.contains(javaVer)) {
      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:${tag}"
    exec {
      executable 'docker'
      workingDir rootDir
      args = [
              'buildx',
              'build',
              '-t',
              imageURL,
              '-f',
              'sdks/java/container/Dockerfile-distroless',
              "--build-arg=BEAM_BASE=gcr.io/apache-beam-testing/beam-sdk/beam_${javaVer}_sdk",
              "--build-arg=DISTROLESS_BASE=gcr.io/distroless/${javaVer}-debian12",
              '.'
      ]
    }
    exec {
      executable 'docker'
      args = ['push', imageURL]
    }
  }
}

Compared to the build file in sdks/java/container/distroless:

docker {
    name containerImageName(
            name: "${base_name}_distroless",
            root: root,
            tag: tag)
    tags containerImageTags()
    dockerfile project.file("../Dockerfile")
    files "./build/"
    buildArgs([
            'BEAM_BASE': base,
            'DISTROLESS_BASE': "gcr.io/distroless/java${imageJavaVersion}-debian12"
    ])
    buildx project.useBuildx()
    platform(*project.containerPlatforms())
    load project.useBuildx() && !pushContainers
    push pushContainers
}

---

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