docker-compose example: do not use depends_on

Wait for the .runner file to exist instead as a proof that the
runner is ready to accept a job

examples/docker-compose/compose-demo-workflow.yml

@@ -7,9 +7,6 @@ services:
     image: alpine:3.19
     links:
       - forgejo
-    depends_on:
-      runner-register:
-        condition: service_completed_successfully
     command: >-
       sh -ec '
       apk add --quiet git curl jq ;