drivers/docker: do not set cgroup parent in v1 mode

This PR fixes a bug where the CgroupParent on the docker
HostConfig struct was accidently being set when running in
cgroups v1 mode.

.changelog/13058.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+docker: Fixed a bug where cgroups-v1 parent was being set
+```

drivers/docker/driver.go

@@ -778,6 +778,15 @@ func memoryLimits(driverHardLimitMB int64, taskMemory drivers.MemoryResources) (
 	return hard * 1024 * 1024, softBytes
 }
 
+// Extract the cgroup parent from the nomad cgroup (only for linux/v2)
+func cgroupParent(resources *drivers.Resources) string {
+	var parent string
+	if cgutil.UseV2 && resources != nil && resources.LinuxResources != nil {
+		parent, _ = cgutil.SplitPath(resources.LinuxResources.CpusetCgroupPath)
+	}
+	return parent
+}
+
 func (d *Driver) createContainerConfig(task *drivers.TaskConfig, driverConfig *TaskConfig,
 	imageID string) (docker.CreateContainerOptions, error) {
 
@@ -843,11 +852,8 @@ func (d *Driver) createContainerConfig(task *drivers.TaskConfig, driverConfig *T
 		pidsLimit = driverConfig.PidsLimit
 	}
 
-	// Extract the cgroup parent from the nomad cgroup (bypass the need for plugin config)
-	parent, _ := cgutil.SplitPath(task.Resources.LinuxResources.CpusetCgroupPath)
-
 	hostConfig := &docker.HostConfig{
-		CgroupParent: parent,
+		CgroupParent: cgroupParent(task.Resources), // if applicable
 
 		Memory:            memory,            // hard limit
 		MemoryReservation: memoryReservation, // soft limit

drivers/docker/driver_test.go

@@ -2844,6 +2844,32 @@ func TestDockerDriver_memoryLimits(t *testing.T) {
 	}
 }
 
+func TestDockerDriver_cgroupParent(t *testing.T) {
+	ci.Parallel(t)
+
+	t.Run("v1", func(t *testing.T) {
+		testutil.CgroupsCompatibleV1(t)
+
+		parent := cgroupParent(&drivers.Resources{
+			LinuxResources: &drivers.LinuxResources{
+				CpusetCgroupPath: "/sys/fs/cgroup/cpuset/nomad",
+			},
+		})
+		require.Equal(t, "", parent)
+	})
+
+	t.Run("v2", func(t *testing.T) {
+		testutil.CgroupsCompatibleV2(t)
+
+		parent := cgroupParent(&drivers.Resources{
+			LinuxResources: &drivers.LinuxResources{
+				CpusetCgroupPath: "/sys/fs/cgroup/nomad.slice",
+			},
+		})
+		require.Equal(t, "nomad.slice", parent)
+	})
+}
+
 func TestDockerDriver_parseSignal(t *testing.T) {
 	ci.Parallel(t)