Docker Swarm Service - force update of latest image already running

Docker won't automatically perform a pull from DockerHub (or private registry) for an image:tag you already have locally.

If you performed a manual pull before the docker service update, or deleted the image locally, it would.

You could also chain the command:

docker pull image:tag && docker service update --image username/imagename:latest servicename

You can avoid this scenario by tagging your images numerically and using an updated tag. username/imagename:1.1.0

Just for future reference:

Docker 1.13 added a --force flag to service update:

--force: Force update even if no changes require it

Use it as in:

docker service update --force service_name