When updates happen, the current behavior is to kill all running
instances and dispatch new tasks. Common use cases for container updates
involve small numbers of containers, meaning the app will go down on
most updates. Setting parallelism to 1 ensures that at most one task
will go down during the update. Services with higher replica counts can
increase this number accordingly to meet their needs.
Signed-off-by: Stephen J Day <stephen.day@docker.com>