web: Use tighter regexp to find container cgroups.

Otherwise we might pick up cgroups like
var-lib-docker-devicemapper-mnt-5cc43057ad4d45b2a1f01a7db64556c912526318eb365d3203535cc0fefc4c9c.mount.
This might cause handle_new_samples to store the wrong numbers in the
container data structure.

The sympton was that the resource graphs would work as expected but
the container rows would always show 0% CPU and "?" for the memory.

More tuning for the regexp might be needed, and ideally Docker should
tell us the cgroup of a container.