Detect docker as virtualization_type

New docker versions use "docker" in /proc/1/cgroup, which disturbs the
virtualization_type detection. So, grep for docker, in addition of
"lxc".

lib/ansible/module_utils/facts.py

@@ -2159,6 +2159,10 @@ class LinuxVirtual(Virtual):
 
         if os.path.exists('/proc/1/cgroup'):
             for line in open('/proc/1/cgroup').readlines():
+                if re.search('/docker/', line):
+                    self.facts['virtualization_type'] = 'docker'
+                    self.facts['virtualization_role'] = 'guest'
+                    return
                 if re.search('/lxc/', line):
                     self.facts['virtualization_type'] = 'lxc'
                     self.facts['virtualization_role'] = 'guest'