dockerd-rootless-setuptool.sh: check RootlessKit functionality

RootlessKit will print hints if something is still unsatisfied.

e.g., `kernel.apparmor_restrict_unprivileged_userns` constraint
rootless-containers/rootlesskit@33c3e7ca6c3f76e74e8acd5b53668dcd3d8e58e0

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit b32cfc3b3a9d791ed3924ab168f27962dceec949)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

contrib/dockerd-rootless-setuptool.sh

@@ -269,6 +269,13 @@ init() {
 	# - sysctl: "net.ipv4.ip_unprivileged_port_start"
 	# - external binary: slirp4netns
 	# - external binary: fuse-overlayfs
+
+	# check RootlessKit functionality. RootlessKit will print hints if something is still unsatisfied.
+	# (e.g., `kernel.apparmor_restrict_unprivileged_userns` constraint)
+	if ! rootlesskit true; then
+		ERROR "RootlessKit failed, see the error messages and https://rootlesscontaine.rs/getting-started/common/ ."
+		exit 1
+	fi
 }
 
 # CLI subcommand: "check"