Don't hardcode Vagrant user

Change-Id: I4ef50359570a3d2fbfe06a0a4e5f9821c781a1b3
diff --git a/ansible/roles/docker/tasks/debian.yml b/ansible/roles/docker/tasks/debian.yml
index aa10934..f3d8b7e 100644
--- a/ansible/roles/docker/tasks/debian.yml
+++ b/ansible/roles/docker/tasks/debian.yml
@@ -37,7 +37,7 @@
 
 - name: vagrant user is added to the docker group
   user:
-    name: vagrant
+    name: "{{ ansible_env['SUDO_USER'] }}"
     group: docker
   register: user_result
   tags: [docker]