[docker] fix container ID extraction for RancherOS

[hkaj]

fixes #2454

Lines in /proc/$PID/cgroup don't have the same format for all system.

• In CoreOS they look like: 3:cpu,cpuacct:/system.slice/docker-$CONTAINER_ID.scope
• In RancherOS: 5:cpu,cpuacct:/docker/$USER_DOCKER_CID/docker/$CONTAINER_ID
• In Ubuntu: 4:cpuacct:/docker/$CONTAINER_ID

To avoid picking the first (wrong) container ID, let's switch from search to re.findall.

[gmmeyer]

LGTM 👍