When container is ran with --parent-cgroup to inherit restrictions from another container, the resulting cgroup looks like :

6:memory:/docker/4193df6bcf5fce75f3fc77f303b2ac06fb664adeb269b959b7ae17b3f8dcf329/14d7240da87b145e4992654c908a8631dbf179abb7f88115ea72743e1192d07d

CGROUP_MATCHER_PATTERN don't catch this and won't be able to extract the container ID