dockerd fails to start with `--iptables=false --ip6tables`

Description

When started with --iptables=false and --ip6tables, dockerd fails with:

failed to start daemon: Error initializing network controller: error obtaining controller instance: failed to register "bridge" driver: cannot create new chains, EnableIPTable is disabled

Reproduce

dockerd --ipv6 --fixed-cidr-v6=fd00::/64 --iptables=false --experimental --ip6tables

Expected behavior

dockerd should start, and create IPv6 rules but not IPv4 rules.

docker version

Client: Docker Engine - Community
 Version:           24.0.6
 API version:       1.43
 Go version:        go1.20.7
 Git commit:        ed223bc
 Built:             Mon Sep  4 12:31:44 2023
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          24.0.6
  API version:      1.43 (minimum version 1.12)
  Go version:       go1.20.7
  Git commit:       1a79695
  Built:            Mon Sep  4 12:31:44 2023
  OS/Arch:          linux/amd64
  Experimental:     true
 containerd:
  Version:          1.6.22
  GitCommit:        8165feabfdfe38c65b599c4993d227328c231fca
 runc:
  Version:          1.1.8
  GitCommit:        v1.1.8-0-g82f18fe
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

docker info

Client: Docker Engine - Community
 Version:    24.0.6
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.11.2
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.21.0
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 5
  Running: 5
  Paused: 0
  Stopped: 0
 Images: 1159
 Server Version: 24.0.6
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Using metacopy: false
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: systemd
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 8165feabfdfe38c65b599c4993d227328c231fca
 runc version: v1.1.8-0-g82f18fe
 init version: de40ad0
 Security Options:
  apparmor
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.2.0-26-generic
 Operating System: Ubuntu 22.04.3 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 8
 Total Memory: 23.46GiB
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: true
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false
 Default Address Pools:
   Base: 172.17.0.0/16, Size: 24
   Base: 172.18.0.0/15, Size: 23
   Base: 172.20.0.0/14, Size: 22
   Base: 172.24.0.0/13, Size: 21
   Base: fd01::/64, Size: 64

Additional Info

The following change should fix it (I'll open a pull request after I write a regression test):

diff --git a/libnetwork/drivers/bridge/setup_ip_tables_linux.go b/libnetwork/drivers/bridge/setup_ip_tables_linux.go
index c98781f3b0..2d3ed57d9d 100644
--- a/libnetwork/drivers/bridge/setup_ip_tables_linux.go
+++ b/libnetwork/drivers/bridge/setup_ip_tables_linux.go
@@ -31,11 +31,6 @@ const (
 )
 
 func setupIPChains(config configuration, version iptables.IPVersion) (natChain *iptables.ChainInfo, filterChain *iptables.ChainInfo, isolationChain1 *iptables.ChainInfo, isolationChain2 *iptables.ChainInfo, retErr error) {
-	// Sanity check.
-	if !config.EnableIPTables {
-		return nil, nil, nil, nil, errors.New("cannot create new chains, EnableIPTable is disabled")
-	}
-
 	hairpinMode := !config.EnableUserlandProxy
 
  	iptable := iptables.GetIptable(version)

Start with libnetwork/drivers/bridge/setup_ip_tables_linux.go and reproduce the failure using the dockerd command shown. Add a regression test for --iptables=false with --ip6tables, then verify that dockerd starts, creates IPv6 rules, and does not create IPv4 rules.