Revert "remove the check for /usr/bin/docker in the setup phase"

this is still helpful when executing these test scripts manually
outside Cockpit CI or in Jenkins (for gating).

Partially reverts commit 5f5a2d5337.

tests/cli/test_compose_tar.sh

@@ -16,6 +16,10 @@ CLI="${CLI:-./src/bin/composer-cli}"
 
 
 rlJournalStart
+    rlPhaseStartSetup
+        rlAssertExists /usr/bin/docker
+    rlPhaseEnd
+
     rlPhaseStartTest "compose start"
         rlAssertEquals "SELinux operates in enforcing mode" "$(getenforce)" "Enforcing"
         UUID=`$CLI compose start example-http-server tar`