Docker Swarm tips & tricks:

How to force Docker image update during restarting a Docker Swarm service?

S=stackname_my_docker_swarm_service
docker service update --image "$(docker service ls | grep -P "\s$S\s" | awk '{ print $5 }' )" --force $S

In variable $S we set Docker Swarm service name. The script automatically fetches currently used image reference (f.e. rather generic name like "my_docker_swarm_service:latest") and forces to re-download from repository by specyfying its name using "--force my_docker_swarm_service:latest" parameter.