/proc/[pid]/cgroup gives the join the process list never had: systemd unit,
container id, runtime. Both cgroup generations, both cgroup drivers, docker,
podman, containerd, crio and lxc. On a 321-process desktop every userland
process attributes; the 172 that do not are kernel threads.
The kernel's id is full-length and the docker inventory reports twelve
characters, so the join is a prefix, not equality. RAF carries the rule.
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