Update docker daemon configuration file

 * According to the official docs files in /usr/lib/systemd/system or
   /lib/systemd/system contain the defaults and should not be modified
 * The recommended method to configure the daemon is to place files in
   /etc/systemd/system/docker.service.d
 * https://docs.docker.com/engine/admin/systemd/

tasks/main.yml

@@ -74,10 +74,9 @@
   when: docker_opts != "" and ansible_distribution_version|version_compare(15.04, '>=')
 
 - name: Ensure docker daemon options used (systemd)
-  lineinfile:
-    dest: /lib/systemd/system/docker.service
-    regexp: "ExecStart=/usr/bin/docker daemon -H fd://"
-    line: ExecStart=/usr/bin/docker daemon -H fd:// $DOCKER_OPTS
+  template:
+    src: docker.conf
+    dest: /etc/systemd/system/docker.service.d/docker.conf
   notify:
     - Reload systemd
     - Restart docker

templates/docker.conf

@@ -0,0 +1,3 @@
+[Service]
+ExecStart=
+ExecStart=/usr/bin/docker daemon -H fd:// $DOCKER_OPTS