fix docker compose detection in fleetctl preview (#21006)

for https://github.com/fleetdm/fleet/issues/21007

CI that uses this is passing now
https://github.com/fleetdm/fleet/actions/runs/10220076886/job/28279696099

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

<!-- Note that API documentation changes are now addressed by the
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- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/Committing-Changes.md#changes-files)
for more information.
- [x] Manual QA for all new/changed functionality

changes/21006-fleetctl-preview

@@ -0,0 +1 @@
+* Fixed a bug in `fleetctl preview` that was causing it to fail if Docker was installed without support for the deprecated `docker-compose` CLI

cmd/fleetctl/preview.go

@@ -74,7 +74,7 @@ func (d dockerCompose) Command(arg ...string) *exec.Cmd {
 
 func newDockerCompose() (dockerCompose, error) {
 	// first, check if `docker compose` is available
-	if err := exec.Command("docker compose").Run(); err == nil {
+	if err := exec.Command("docker", "compose").Run(); err == nil {
 		return dockerCompose{dockerComposeV2}, nil
 	}