Closes#47111
is_container() only recognized Docker (/.dockerenv), Podman
(/run/.containerenv), and docker/podman/lxc markers in /proc/1/cgroup.
Under cgroup v2 (Kubernetes/k3s on containerd or CRI-O) /proc/1/cgroup
collapses to a single "0::/" line with no runtime marker, so
is_container() returned False on every containerd/CRI pod.
That false negative bypassed container-aware behavior across the CLI.
The most damaging case (reported): even after #46290 fixed
detect_service_manager() to gate on _s6_running() alone, other
is_container() call sites (profile home resolution, gateway behaviors,
config, doctor) still misbehave on containerd.
Broaden detection conservatively:
- KUBERNETES_SERVICE_HOST env var (present in every k8s pod).
- kubepods/containerd/crio markers in /proc/1/cgroup (cgroup v1 nested).
- same markers in /proc/self/mountinfo as a cgroup-v2 fallback.
Tests: 3 new (k8s env, kubepods cgroup, cgroup-v2-via-mountinfo) plus the
existing negative case hardened to stub mountinfo + env; 108 constants +
service_manager tests pass.
