Docker does not provide the rootfs path through docker inspect or statefile
and the path is dependent on the storage driver being used.
Instead of enumerating the storage drivers, we pick a pid from the container
and get the config from /proc/pid/root. Although a bit expensive, this method
works for non-docker containers too.
The stats are only populated when cAdvisor is running outside network namespaces.
We'll add a different backend to retrieve the same data from within namespaces.
This adds an Exists() interface to detect when the container is dead.
Before reporting an update error we check is Exists() is true.
Some documentation was added as well.
