add gitlab-runner user to the docker group

The user named "gitlab-runner" has to be in the "docker" group if it
hopes to use the docker executor.

roles/gitlab-runner/tasks/main.yml

@@ -59,3 +59,12 @@
   template:
     src: gitlab-runner.toml
     dest: /etc/gitlab-runner/config.toml
+
+- name: Put gitlab-runner in the docker group
+  user:
+    append: yes ## Don't set these as our *only* groups!
+    groups: docker
+    name: "{{ item }}"
+  with_items:
+    - gitlab-runner
+  when: ansible_local.gitlab_runner.executor == "docker"