dockerd-rootless.sh: set `rootlesskit --state-dir=DIR`

Now the state dir is set to `${XDG_RUNTIME_DIR}/dockerd-rootless`.

This is similar to `${XDG_RUNTIME_DIR}/containerd-rootless` used in nerdctl:
https://github.com/containerd/nerdctl/blob/v1.7.2/extras/rootless/containerd-rootless.sh#L35

Prior to this commit, the state dir was unset and a random dir under `/tmp` was used.
(e.g., `/tmp/rootlesskit1869901982`)

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

contrib/dockerd-rootless.sh

@@ -9,6 +9,7 @@
 # * Either one of slirp4netns (>= v0.4.0), VPNKit, lxc-user-nic needs to be installed.
 #
 # Recognized environment variables:
+# * DOCKERD_ROOTLESS_ROOTLESSKIT_STATE_DIR=DIR: the rootlesskit state dir. Defaults to "$XDG_RUNTIME_DIR/dockerd-rootless".
 # * DOCKERD_ROOTLESS_ROOTLESSKIT_NET=(slirp4netns|vpnkit|pasta|lxc-user-nic): the rootlesskit network driver. Defaults to "slirp4netns" if slirp4netns (>= v0.4.0) is installed. Otherwise defaults to "vpnkit".
 # * DOCKERD_ROOTLESS_ROOTLESSKIT_MTU=NUM: the MTU value for the rootlesskit network driver. Defaults to 65520 for slirp4netns, 1500 for other drivers.
 # * DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=(builtin|slirp4netns|implicit): the rootlesskit port driver. Defaults to "builtin".
@@ -64,6 +65,7 @@ if [ -z "$rootlesskit" ]; then
 	exit 1
 fi
 
+: "${DOCKERD_ROOTLESS_ROOTLESSKIT_STATE_DIR:=$XDG_RUNTIME_DIR/dockerd-rootless}"
 : "${DOCKERD_ROOTLESS_ROOTLESSKIT_NET:=}"
 : "${DOCKERD_ROOTLESS_ROOTLESSKIT_MTU:=}"
 : "${DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER:=builtin}"
@@ -119,6 +121,7 @@ if [ -z "$_DOCKERD_ROOTLESS_CHILD" ]; then
 	#         (by either systemd-networkd or NetworkManager)
 	# * /run: copy-up is required so that we can create /run/docker (hardcoded for plugins) in our namespace
 	exec $rootlesskit \
+		--state-dir=$DOCKERD_ROOTLESS_ROOTLESSKIT_STATE_DIR \
 		--net=$net --mtu=$mtu \
 		--slirp4netns-sandbox=$DOCKERD_ROOTLESS_ROOTLESSKIT_SLIRP4NETNS_SANDBOX \
 		--slirp4netns-seccomp=$DOCKERD_ROOTLESS_ROOTLESSKIT_SLIRP4NETNS_SECCOMP \