tests/docker.go: explicitly configure btrfs driver in daemon.json

This change adds an explicit brtfs driver request in
/etc/docker/daemon.json to un-break the docker btrfs test for docker 23
and above. The explicit storage driver configuration does not break
docker 20 and older; it works fine if btrfs storage is provided (which
it is in the test).

Starting with docker 23, overlay2 is the default also for btrfs
filesystems, and the btrfs driver is opt-in. See
https://docs.docker.com/engine/release-notes/23.0/#bug-fixes-and-enhancements-6
and
moby/moby#42661

Signed-off-by: Thilo Fromm <thilofromm@microsoft.com>

kola/tests/docker/docker.go

@@ -142,11 +142,24 @@ passwd:
 		// This test is normally not related to the cloud environment
 		Platforms: []string{"qemu", "qemu-unpriv"},
 		// Note: copied verbatim from https://github.com/coreos/docs/blob/master/os/mounting-storage.md#creating-and-mounting-a-btrfs-volume-file
-		UserData: conf.ContainerLinuxConfig(`
+		// Added explicit btrfs driver selection because overlay2 is the default for btrfs FS in docker 23 and above
+		UserData: conf.Butane(`
+variant: flatcar
+version: 1.0.0
+
+storage:
+  files:
+  - path: /etc/docker/daemon.json
+    contents:
+      inline: |
+        {
+          "storage-driver": "btrfs"
+        }
+
 systemd:
   units:
     - name: format-var-lib-docker.service
-      enable: true
+      enabled: true
       contents: |
         [Unit]
         Before=docker.service var-lib-docker.mount
@@ -158,7 +171,7 @@ systemd:
         [Install]
         WantedBy=multi-user.target
     - name: var-lib-docker.mount
-      enable: true
+      enabled: true
       contents: |
         [Unit]
         Before=docker.service