fix(bootstrap): support cgroup v1 hosts by disabling kubelet failCgroupV1 check (#360)

Kubernetes 1.35 (k3s v1.35.x) rejects cgroup v1 by default via the
failCgroupV1 kubelet config option. This breaks gateway startup on
distros that still use cgroup v1 (e.g. Rocky Linux 8, CentOS 7/8).

Detect cgroup v1 at container startup by checking for the absence of
/sys/fs/cgroup/cgroup.controllers and pass
--kubelet-arg=fail-cgroupv1=false so kubelet warns instead of exiting.

Author: drew

deploy/docker/cluster-entrypoint.sh

@@ -482,10 +482,25 @@ fi
 # Pre-creating the directory eliminates this failure mode.
 mkdir -p /run/flannel
 
+# ---------------------------------------------------------------------------
+# Detect cgroup version and set kubelet compatibility flags
+# ---------------------------------------------------------------------------
+# Kubernetes 1.35+ (k3s v1.35.x) rejects cgroup v1 by default. Hosts running
+# older distros (e.g. Rocky Linux 8, CentOS 7/8, Ubuntu 18.04) still use
+# cgroup v1. When we detect cgroup v1, pass --kubelet-arg=fail-cgroupv1=false
+# so kubelet warns instead of refusing to start. This flag can be removed once
+# cgroup v1 support is no longer needed.
+EXTRA_KUBELET_ARGS=""
+if [ ! -f /sys/fs/cgroup/cgroup.controllers ]; then
+    echo "Detected cgroup v1 — adding kubelet compatibility flag (fail-cgroupv1=false)"
+    EXTRA_KUBELET_ARGS="--kubelet-arg=fail-cgroupv1=false"
+fi
+
 # Docker Desktop can briefly start the container before its bridge default route
 # is fully installed. k3s exits immediately in that state, so wait briefly for
 # routing to settle first.
 wait_for_default_route
 
 # Execute k3s with explicit resolv-conf.
-exec /bin/k3s "$@" --resolv-conf="$RESOLV_CONF"
+# shellcheck disable=SC2086
+exec /bin/k3s "$@" --resolv-conf="$RESOLV_CONF" $EXTRA_KUBELET_ARGS