Use of root cgroup v2 yields missing features

Description

With cgroup v2, some features merely exist in non-root cgroups, per documentation; for instance, memory.swap.high & memory.swap.max defining swap limits.

Hence running dockerd on systems with cgroup v2 makes swap limits unavailable.

Reproduce

1. docker info | grep swap

   WARNING: No swap limit support
   

2. docker create --name demo-swap --memory 6m busybox:latest

   WARNING: Your kernel does not support swap limit capabilities or the cgroup is not mounted. Memory limited without swap.
   

Expected behavior

All cgroup v2 capabilities available (non-root cgroup)

docker version

Client: Docker Engine - Community
 Version:           29.0.0
 API version:       1.52
 Go version:        go1.25.4
 Git commit:        3d4129b
 Built:             Mon Nov 10 21:46:28 2025
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          29.0.0
  API version:      1.52 (minimum version 1.44)
  Go version:       go1.25.4
  Git commit:       d105562
  Built:            Mon Nov 10 21:46:28 2025
  OS/Arch:          linux/amd64
  Experimental:     true
 containerd:
  Version:          v2.1.5
  GitCommit:        fcd43222d6b07379a4be9786bda52438f0dd16a1
 runc:
  Version:          1.3.3
  GitCommit:        v1.3.3-0-gd842d771
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

docker info

Client: Docker Engine - Community
 Version:    29.0.0
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.29.1
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.40.3
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 24
  Running: 21
  Paused: 0
  Stopped: 3
 Images: 45
 Server Version: 29.0.0
 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: cgroupfs
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 CDI spec directories:
  /etc/cdi
  /var/run/cdi
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: fcd43222d6b07379a4be9786bda52438f0dd16a1
 runc version: v1.3.3-0-gd842d771
 init version: de40ad0
 Security Options:
  apparmor
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 6.12.48+deb13-amd64
 Operating System: Devuan GNU/Linux 6 (excalibur)
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 7.732GiB
 Name: <***>
 ID: IHQO:MIY4:BIHW:K7ZF:FD74:NWSG:A3ZE:EDAU:3QJU:PMER:A2HH:GA5P
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: true
 Insecure Registries:
  ::1/128
  127.0.0.0/8
 Live Restore Enabled: true
 Firewall Backend: iptables

WARNING: No swap limit support

Additional Info

No response

No source files or tests are named in the issue. Start by tracing Docker's cgroup v2 capability detection and how it handles the root cgroup, then compare that behavior with a non-root cgroup. Done means docker info reports swap-limit support and creating a memory-limited container no longer warns that swap limits are unavailable.