Major Description
DockerContainerizer doesn't accept tasks that do not have TaskInfo.ContainerInfo set, but MesosContainerizer accepts tasks even if TaskInfo.ContainerInfo is set. It should not.
This means, currently if a slave has to support both docker and non-docker tasks, the order of containerizers in --containerizers is important, viz., "docker,mesos" works but "mesos,docker" doesn't work when running a non-docker task.