MT#22099 Docker >=1.11 no longer has /.dockerinit, adjust docker run check accordingly

File /.dockerinit no longer exists. With Docker versions 1.11 and
1.12 there's /.dockerenv available, though it's not yet sure
whether this will be present in the future too, so check for either
/.dockerenv or a proper cgroup setup inside the docker container.
File /proc/1/cgroup with Docker version 1.12.1 on Debian/jessie
looks like:

| root@4c3a1c6d773c:/# cat /proc/1/cgroup
| 8:freezer:/docker/4c3a1c6d773c1a5675f24ff58540ff1a290489e7325e1e66ec2c3392aa3a1145
| 7:perf_event:/docker/4c3a1c6d773c1a5675f24ff58540ff1a290489e7325e1e66ec2c3392aa3a1145
| 6:net_cls,net_prio:/docker/4c3a1c6d773c1a5675f24ff58540ff1a290489e7325e1e66ec2c3392aa3a1145
| 5:cpuset:/docker/4c3a1c6d773c1a5675f24ff58540ff1a290489e7325e1e66ec2c3392aa3a1145
| 4:blkio:/docker/4c3a1c6d773c1a5675f24ff58540ff1a290489e7325e1e66ec2c3392aa3a1145
| 3:cpu,cpuacct:/docker/4c3a1c6d773c1a5675f24ff58540ff1a290489e7325e1e66ec2c3392aa3a1145
| 2:devices:/docker/4c3a1c6d773c1a5675f24ff58540ff1a290489e7325e1e66ec2c3392aa3a1145
| 1:name=systemd:/docker/4c3a1c6d773c1a5675f24ff58540ff1a290489e7325e1e66ec2c3392aa3a1145

See https://github.com/docker/docker/issues/18355 and
https://github.com/docker/docker/pull/19490 for details.

Change-Id: I2d649f65c8bba50193171ec7fbfd20160ddfc319

t/testrunner

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-if ! [ -f /.dockerinit ]; then
+if ! [ -f /.dockerenv ] && ! grep -q 'devices:/docker' /proc/1/cgroup ; then
   echo "Not running inside docker, exiting to avoid data damage." >&2
   exit 1
 fi