web: Be more flexible in how we expect container cgroups

The container cgroups we expect are pretty brittle. Upstream hasn't
yet figured out how they want to do it, leading to all sorts of
strange cgroup names.

Just look for container ids (64-hex-characters) in the cgroups, and
try to find matching containers based on that.

Closes #414
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>