Take subvol mount option for root into account

If the rootfs is btrfs based make sure the fstab entry for
it takes the name of the root subvolume into account

kiwi/builder/disk.py

@@ -1119,6 +1119,13 @@ def _write_generic_fstab(
             custom_root_mount_args += ['ro']
             fs_check_interval = '0 0'
 
+        if self.volume_manager_name and self.volume_manager_name == 'btrfs':
+            root_volume_name = system.get_root_volume_name()
+            if root_volume_name != '/':
+                custom_root_mount_args += [
+                    f'defaults,subvol={root_volume_name}'
+                ]
+
         self._add_fstab_entry(
             device_map['root'].get_device(), '/',
             custom_root_mount_args, fs_check_interval

test/unit/builder/disk_test.py

@@ -1197,6 +1197,41 @@ def get_filesystem():
             call('LABEL=blkid_result /var ext3 defaults 0 0')
         ] in self.disk_builder.fstab.add_entry.call_args_list
 
+    @patch('kiwi.builder.disk.FileSystem.new')
+    @patch('kiwi.builder.disk.VolumeManager.new')
+    @patch('kiwi.builder.disk.Command.run')
+    @patch('kiwi.builder.disk.Defaults.get_grub_boot_directory_name')
+    @patch('kiwi.builder.disk.ImageSystem')
+    @patch('os.path.exists')
+    def test_create_disk_btrfs_managed_root(
+        self, mock_exists, mock_ImageSystem, mock_grub_dir, mock_command,
+        mock_volume_manager, mock_fs
+    ):
+        mock_exists.return_value = True
+        volume_manager = Mock()
+        volume_manager.get_device = Mock(
+            return_value={
+                'root': MappedDevice('root', Mock())
+            }
+        )
+        volume_manager.get_fstab = Mock(
+            return_value=['fstab_volume_entries']
+        )
+        volume_manager.get_root_volume_name = Mock(
+            return_value='@'
+        )
+        mock_volume_manager.return_value = volume_manager
+        filesystem = Mock()
+        mock_fs.return_value = filesystem
+        self.disk_builder.volume_manager_name = 'btrfs'
+
+        with patch('builtins.open'):
+            self.disk_builder.create_disk()
+
+        assert [
+            call('UUID=blkid_result / blkid_result_fs ro,defaults,subvol=@ 0 0')
+        ] in self.disk_builder.fstab.add_entry.call_args_list
+
     @patch('kiwi.builder.disk.FileSystem.new')
     @patch('kiwi.builder.disk.VolumeManager.new')
     @patch('kiwi.builder.disk.Command.run')