I’m not sure if I completely follow what you are trying to do, but here’s what I think would work in your use case.
Launch a service that has
io.rancher.container.start_once: 'true' in it.
Assuming your container will automatically exit (as you would use the label so that after it exits, it wouldn’t try to launch another container), the state of the service would be
Started-Once in the UI.
In the UI, you can click on the container in the service and select “Start” to have the container start up again and read the output log, but you’d have to read it before the container stops again.
If your container is not going to automatically stop and exit, then you don’t need the label. You can create the service using
rancher-compose create, which creates them in the UI but does not start them. Then, when you are ready to run the test, you could start the service in the UI and read the output log.