dockerd-rootless-setuptool.sh: add `nsenter` subcommand

Usage: `dockerd-rootless-setuptool.sh nsenter -- ip a`

Expected to be used for debugging.

Ported from nerdctl's `containerd-rootless-setuptool.sh`
https://github.com/containerd/nerdctl/blob/v1.7.2/extras/rootless/containerd-rootless-setuptool.sh#L142-L147

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

contrib/dockerd-rootless-setuptool.sh

@@ -273,10 +273,18 @@ init() {
 
 # CLI subcommand: "check"
 cmd_entrypoint_check() {
+	init
 	# requirements are already checked in init()
 	INFO "Requirements are satisfied"
 }
 
+# CLI subcommand: "nsenter"
+cmd_entrypoint_nsenter() {
+	# No need to call init()
+	pid=$(cat "$XDG_RUNTIME_DIR/dockerd-rootless/child_pid")
+	exec nsenter --no-fork --wd="$(pwd)" --preserve-credentials -m -n -U -t "$pid" -- "$@"
+}
+
 show_systemd_error() {
 	n="20"
 	ERROR "Failed to start ${SYSTEMD_UNIT}. Run \`journalctl -n ${n} --no-pager --user --unit ${SYSTEMD_UNIT}\` to show the error log."
@@ -383,6 +391,7 @@ cli_ctx_rm() {
 
 # CLI subcommand: "install"
 cmd_entrypoint_install() {
+	init
 	# requirements are already checked in init()
 	if [ -z "$SYSTEMD" ]; then
 		install_nonsystemd
@@ -416,6 +425,7 @@ cmd_entrypoint_install() {
 
 # CLI subcommand: "uninstall"
 cmd_entrypoint_uninstall() {
+	init
 	# requirements are already checked in init()
 	if [ -z "$SYSTEMD" ]; then
 		INFO "systemd not detected, ${DOCKERD_ROOTLESS_SH} needs to be stopped manually:"
@@ -461,6 +471,7 @@ usage() {
 	echo
 	echo "Commands:"
 	echo "  check        Check prerequisites"
+	echo "  nsenter      Enter into RootlessKit namespaces (mostly for debugging)"
 	echo "  install      Install systemd unit (if systemd is available) and show how to manage the service"
 	echo "  uninstall    Uninstall systemd unit"
 }
@@ -508,5 +519,4 @@ if ! command -v "cmd_entrypoint_${command}" > /dev/null 2>&1; then
 fi
 
 # main
-init
-"cmd_entrypoint_${command}"
+"cmd_entrypoint_${command}" "$@"