docs for Fedora 31 users

BuildKit with crun works fine on cgroup2 system.
Tested both Rootful and Rootless on Fedora 31, with crun v0.10.2.

Signed-off-by: Akihiro Suda <[email protected]>

README.md

@@ -94,7 +94,7 @@ BuildKit is composed of the `buildkitd` daemon and the `buildctl` client.
 While the `buildctl` client is available for Linux, macOS, and Windows, the `buildkitd` daemon is only available for Linux currently.
 
 The `buildkitd` daemon requires the following components to be installed:
--   [runc](https://github.com/opencontainers/runc)
+-   [runc](https://github.com/opencontainers/runc) or [crun](https://github.com/containers/crun)
 -   [containerd](https://github.com/containerd/containerd) (if you want to use containerd worker)
 
 The latest binaries of BuildKit are available [here](https://github.com/moby/buildkit/releases) for Linux, macOS, and Windows.
@@ -125,6 +125,11 @@ We are open to adding more backends.
 The buildkitd daemon listens gRPC API on `/run/buildkit/buildkitd.sock` by default, but you can also use TCP sockets.
 See [Expose BuildKit as a TCP service](#expose-buildkit-as-a-tcp-service).
 
+:information_source: Notice to Fedora 31 users:
+
+* As runc still does not work on cgroup v2 environment like Fedora 31, you need to substitute runc with crun. Run `rm -f $(which buildkit-runc) && ln -s $(which crun) /usr/local/bin/buildkit-runc` .
+* If you want to use runc, you need to configure the system to use cgroup v1. Run `sudo grubby --update-kernel=ALL --args="systemd.unified_cgroup_hierarchy=0"` and reboot.
+
 ### Exploring LLB
 
 BuildKit builds are based on a binary intermediate format called LLB that is used for defining the dependency graph for processes running part of your build. tl;dr: LLB is to Dockerfile what LLVM IR is to C.

docs/rootless.md

@@ -17,7 +17,8 @@ Using Ubuntu kernel is recommended.
 * Add `kernel.unprivileged_userns_clone=1` to `/etc/sysctl.conf` (or `/etc/sysctl.d`) and run `sudo sysctl -p`
 
 ### Fedora 31 and later
-* Run `sudo grubby --update-kernel=ALL --args="systemd.unified_cgroup_hierarchy=0"` and reboot.
+* As runc still does not work on cgroup v2 environment like Fedora 31, you need to substitute runc with crun. Run `rm -f $(which buildkit-runc) && ln -s $(which crun) /usr/local/bin/buildkit-runc` .
+* If you want to use runc, you need to configure the system to use cgroup v1. Run `sudo grubby --update-kernel=ALL --args="systemd.unified_cgroup_hierarchy=0"` and reboot.
 
 ### Fedora 30
 * No preparation is needed