Added docker itself.

mariadb.yml

@@ -4,10 +4,7 @@
   hosts: all # Should be specified of course
   become: True
   tasks:
-    - name: install docker packages
+    - include: tasks/docker.yml
-      apt: pkg={{ item }} state=latest update_cache=yes
-      with_items:
-         - docker-engine
     - name: install service file.
       template:
         src: files/mysql.service
@@ -19,5 +16,5 @@
       command: systemctl daemon-reload
     - name: make sure service is started
       systemd:
-        name: mariadb.service
+        name: mysql.service
         state: started

tasks/docker.yml

@@ -0,0 +1,12 @@
+---
+- apt_key:
+    id: 58118E89F3A912897C070ADBF76221572C52609D
+    keyserver: hkp://p80.pool.sks-keyservers.net:80
+    state: present
+
+- apt_repository:
+    repo: deb https://apt.dockerproject.org/repo ubuntu-xenial main
+    update_cache: yes
+
+- apt:
+    name: docker-engine