make /sys/fs/cgroup rw by default with cgroup2

[agowa]

Description

Make /sys/fs/cgroup rw by default within the container if cgroup2 is used. Currently running e.g. systemd within the container fails because it cannot create it's cgroups as /sys/fs/cgroup is mounted ro. Also now with cgroup2 being supported the old container escapes of v1 are no longer an issue.

For anyone wanting a q'n'd fix for this problem I've attached: #42043

Steps to reproduce the issue:
1.
2.
3.

Describe the results you received:

Describe the results you expected:
Applications within the container being able to create child cgroups and "manage" there own assigned scope.

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Docker version 20.10.3, build 48d30b5b32

Output of docker info:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  app: Docker App (Docker Inc., v0.9.1-beta3)
  buildx: Build with BuildKit (Docker Inc., v0.5.1-tp-docker)

Server:
 Containers: 1
  Running: 1
  Paused: 0
  Stopped: 0
 Images: 57
 Server Version: 20.10.3
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: 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 io.containerd.runtime.v1.linux runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 269548fa27e0089a8b8278fc4fc781d7f65a939b.m
 runc version: 12644e614e25b05da6fd08a38ffa0cfe1903fdec
 init version: de40ad0
 Security Options:
  seccomp
   Profile: default
  cgroupns
 Kernel Version: 5.10.16-arch1-1
 Operating System: Arch Linux
 OSType: linux
 Architecture: x86_64
 CPUs: 16
 Total Memory: 27.11GiB
 Name: ArchBook
 ID: ARJ3:2WKK:VWM6:MEG5:SVQF:NKWZ:SVQQ:2KNQ:P7WI:MG3W:SGX6:3TX6
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Username: agowa338
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

Additional environment details (AWS, VirtualBox, physical, etc.):

[eugene-bright]

The workaround I found for myself:

--cgroupns=private --cgroup-parent=myservice.slice -v /sys/fs/cgroup/myservice.slice:/sys/fs/cgroup