examples/docker-compose: fix false positive

because of the -x the success string was always in the output and the
workflow .forgejo/workflows/example-docker-compose.yml would always
report success, even if it fails

.forgejo/workflows/example-docker-compose.yml

@@ -42,11 +42,13 @@ jobs:
           #
           $cli ps --help
           success='DEMO WORKFLOW SUCCESS'
+          failure='DEMO WORKFLOW FAILURE'
           for delay in $(seq 60) ; do
             $cli logs -n 10 demo-workflow > /tmp/out
             grep --quiet "$success" /tmp/out && break
+            grep --quiet "$failure" /tmp/out && break
             $cli ps --all
-            cat /tmp/out
+            tail /tmp/out
             sleep 10
           done
           if ! grep --quiet "$success" /tmp/out ; then