Already found the solution.
My image is hosted on a private repository.
Besides the swarm manager (where I executed the commands), I had a running swarm worker.
When I ran docker stack deploy -c docker-compose.yml myapp docker deployed the service to the worker node (not the manager node as I thought).
At the worker node, docker had no credentials to pull the image from the private repository.
Hence, to fix this either pass the flag --with-registry-auth (which pushes the credentials for the repository to the worker node) or make sure that the service is deployed to a node where the image is present.
See: https://docs.docker.com/engine/reference/commandline/deploy/