[docker] check considers docker daemon as a container on old docker versions

[hkaj]

this only affects older docker versions

In older docker versions /proc/$DOCKER_DAEMON_PID/cgroup looks like

10:net_prio:/docker-daemon
9:perf_event:/docker-daemon
8:blkio:/docker-daemon
7:net_cls:/docker-daemon
6:freezer:/docker-daemon
5:devices:/docker-daemon
4:memory:/docker-daemon
3:cpuacct:/docker-daemon
2:cpu:/docker-daemon
1:cpuset:/docker-daemon

In the code we consider the process as a container if 'docker' in line[2] here: https://github.com/DataDog/dd-agent/blob/5.8.5/checks.d/docker_daemon.py#L807

Recent versions are not affected since the lines look like i:whatever:/, without the matching docker string.

This was discovered on a docker daemon version 1.9.1 and does not exist in 1.11 so it should affect few people but should still be fixed.

[hkaj]

Fixed by #2847