dockerd.md: Add --firewall-backend

Related to moby/moby@39ab393

Signed-off-by: Rob Murray <rob.murray@docker.com>

Author: robmry

docs/reference/dockerd.md

@@ -59,6 +59,7 @@ Options:
       --exec-root string                      Root directory for execution state files (default "/var/run/docker")
       --experimental                          Enable experimental features
       --feature map                           Enable feature in the daemon
+      --firewall-backend string               Firewall backend to use, iptables or nftables
       --fixed-cidr string                     IPv4 subnet for the default bridge network
       --fixed-cidr-v6 string                  IPv6 subnet for the default bridge network
   -G, --group string                          Group for the unix socket (default "docker")
@@ -1122,6 +1123,7 @@ The following is a full example of the allowed configuration options on Linux:
     "cdi": true,
     "containerd-snapshotter": true
   },
+  "firewall-backend": "",
   "fixed-cidr": "",
   "fixed-cidr-v6": "",
   "group": "",