[Bug]: `is_container()` false-positives on hosts running Docker containers (containerd markers in mountinfo), making `home_mode: auto` subprocess HOME non-deterministic — breaks browser auto-launch with "Chrome not found"

is_container() misidentifies a host as running inside a container when Docker containers using the containerd snapshotter are present, because it scans mountinfo for markers that also appear in overlay mounts on the host; the cached false result destabilizes home_mode: auto and can break browser auto-launch.