Feature idea

I belive docker service is missing a way to exec on all replicas of service.

The reason is, that if you want to for example reload config file you updated in volume, you have to restart service inside container. Without this feature you have to remove the service and create new, which cause downtime...

Steps to reproduce the issue:

docker service update --exec ...

This command would exec command on all replicas of service. --update-parallelism and update-delay would affect parallelism and delay of this action