tcwg-docker: Allow tcwg-benchmark user direct access to docker.

We need this to start docker build containers from benchmarking job.

Change-Id: I4d9893ddf58bde1b90128eff6596f3e743200e18
diff --git a/playbooks/roles/tcwg-docker/tasks/main.yml b/playbooks/roles/tcwg-docker/tasks/main.yml
index 8dfb9f6..9289fab 100644
--- a/playbooks/roles/tcwg-docker/tasks/main.yml
+++ b/playbooks/roles/tcwg-docker/tasks/main.yml
@@ -13,6 +13,9 @@
    notify: restart docker
    when: ansible_cmdline.tegraid is defined
 
+ - name: Append tcwg-benchmark to docker group
+   user: name=tcwg-benchmark groups=docker append=yes
+
  - name: Append tcwg-buildslave to docker group
    user: name=tcwg-buildslave groups=docker append=yes