fix(cli): improve container detection when cgroupns=private (#15156)

Fixes https://github.com/coder/coder/issues/12721

If a container in docker is started with `--cgroupns=private` (which is
the default behaviour in docker) then `/proc/1/cgroup` has the following
content:
```
0::/
```

If a container in docker is started with `--cgroupns=host` then
`/proc/1/cgroup` has the following content (hash will vary):
```
0::/docker/aa86ac98959eeedeae0ecb6e0c9ddd8ae8b97a9d0fdccccf7ea7a474f4e0bb1f
```

Currently we are determining if a host is containerized by assuming the
second scenario. This means the existing behaviour of sniffing
`/proc/1/cgroup` is not always sufficient for checking if a host is
containerized.

According to [the cgroups(7)
man-page](https://man7.org/linux/man-pages/man7/cgroups.7.html) there
exists a `cgroup.type` file in a nonroot cgroup. This exists in Linux
versions after `4.14`.

> Linux 4.14 added thread mode for cgroups v2.

> With the addition of thread mode, each nonroot cgroup now contains a
new file, cgroup.type

This means we can check for the existence of
`/sys/fs/cgroup/cgroup.type` to see if we are in a container or not.

cli/clistat/container.go

@@ -12,6 +12,7 @@ import (
 const (
 	procMounts                           = "/proc/mounts"
 	procOneCgroup                        = "/proc/1/cgroup"
+	sysCgroupType                        = "/sys/fs/cgroup/cgroup.type"
 	kubernetesDefaultServiceAccountToken = "/var/run/secrets/kubernetes.io/serviceaccount/token" //nolint:gosec
 )
 
@@ -65,6 +66,17 @@ func IsContainerized(fs afero.Fs) (ok bool, err error) {
 		}
 	}
 
+	// Adapted from https://github.com/systemd/systemd/blob/88bbf187a9b2ebe0732caa1e886616ae5f8186da/src/basic/virt.c#L603-L605
+	// The file `/sys/fs/cgroup/cgroup.type` does not exist on the root cgroup.
+	// If this file exists we can be sure we're in a container.
+	cgTypeExists, err := afero.Exists(fs, sysCgroupType)
+	if err != nil {
+		return false, xerrors.Errorf("check file exists %s: %w", sysCgroupType, err)
+	}
+	if cgTypeExists {
+		return true, nil
+	}
+
 	// If we get here, we are _probably_ not running in a container.
 	return false, nil
 }

cli/clistat/stat_internal_test.go

@@ -309,6 +309,12 @@ func TestIsContainerized(t *testing.T) {
 			Expected: true,
 			Error:    "",
 		},
+		{
+			Name:     "Docker (Cgroupns=private)",
+			FS:       fsContainerCgroupV2PrivateCgroupns,
+			Expected: true,
+			Error:    "",
+		},
 	} {
 		tt := tt
 		t.Run(tt.Name, func(t *testing.T) {
@@ -374,6 +380,12 @@ proc /proc/sys proc ro,nosuid,nodev,noexec,relatime 0 0`,
 		cgroupV2MemoryUsageBytes: "536870912",
 		cgroupV2MemoryStat:       "inactive_file 268435456",
 	}
+	fsContainerCgroupV2PrivateCgroupns = map[string]string{
+		procOneCgroup: "0::/",
+		procMounts: `overlay / overlay rw,relatime,lowerdir=/some/path:/some/path,upperdir=/some/path:/some/path,workdir=/some/path:/some/path 0 0
+proc /proc/sys proc ro,nosuid,nodev,noexec,relatime 0 0`,
+		sysCgroupType: "domain",
+	}
 	fsContainerCgroupV1 = map[string]string{
 		procOneCgroup: "0::/docker/aa86ac98959eeedeae0ecb6e0c9ddd8ae8b97a9d0fdccccf7ea7a474f4e0bb1f",
 		procMounts: `overlay / overlay rw,relatime,lowerdir=/some/path:/some/path,upperdir=/some/path:/some/path,workdir=/some/path:/some/path 0 0