Title: safer example runner

Description:

I just used envoy/examples/verify-common.sh and during its cleanup it does a docker prune, which removes ALL stopped containers. I have dozens of containers in the stop state and use them for various unrelated activities. While I did read the script, I didn't put it together that "verify-...." was going to remove things it didn't own. I recommend injecting id information on these related containers and only delete them. Maybe via ENV, or something like that. If others think that is a good idea (or maybe just adding documentation or a feature gate on delete) I am open to implementing it.