How can you modify the number of replicas for a service in Docker Swarm?

To modify the number of replicas for a service in Docker Swarm, you use the command designed specifically for scaling services, which is docker service scale. This command allows you to specify the desired number of replicas directly in the command line interface. For example, using the command docker service scale my_service=5 would adjust the replicas for the service named my_service to five.

Adjusting service parameters in the Dockerfile is not relevant for scaling, as the Dockerfile is used primarily to define how an image is built rather than how a service runs in Swarm. The Docker Swarm UI, if available, can provide a graphical interface for managing services, but it is often not as straightforward and not all versions have it. Lastly, docker scale replicas is not a valid Docker command; the correct syntax requires specifying the service name. Thus, the appropriate method for scaling services in Docker Swarm is through the docker service scale command.