gradle: Pin version as current latest image is broken.

Signed-off-by: Matthieu Nottale <matthieu.nottale@docker.com>

Dockerfile.gradle

@@ -1,3 +1,3 @@
-FROM gradle:jdk8
+FROM gradle:4.9-jdk8
 COPY bin/docker-app-linux /usr/local/bin/docker-app
 COPY --chown=gradle:gradle integrations/gradle .