Use docker api in order to find container pid

Previously, the docker driver would try to obtain a containers pid by
looking it up in /sys/fs/cgroup/drivces/ though this path is not always
correct depending on the version of docker one is using. This patch
corrects this by getting the pid by querying docker instead.

Change-Id: If3cc342dc781a360f2414a1291efda57d102a85b
Closes-bug: 1292479
Co-Authored-By: Pavel Sviderski <pavel@psviderski.name>