Adds container stat logging.

Adds container stat logging before termininating container to help aid debugging
docker issue related to "unregister_netdevice: waiting for ...". See
moby/moby#5618 for more details.

helios-services/src/main/java/com/spotify/helios/agent/TaskRunner.java

@@ -115,6 +115,13 @@ public void stop() throws InterruptedException {
     // Interrupt the thread blocking on waitContainer
     stopAsync().awaitTerminated();
 
+    if (System.getenv("CONTAINER_STATS") != null) {
+      try {
+        log.info("container {} stats: {}", containerName, docker.stats(containerName));
+      } catch (DockerException e) {
+        log.warn("Could not log container stats. Exception was {}", e);
+      }
+    }
     try {
       docker.stopContainer(container, secondsToWaitBeforeKill);
     } catch (DockerException e) {