https://github.com/moby/moby/issues/42786

stack.dev.yaml

version: "3.3"

services:
  caddy:
    deploy:
      # this cannot control the number of replicas
      replicas: 0

  mongo:
    ports:
      - 127.0.0.1:27017:27017
    deploy:
      # this can control the number of replicas
      replicas: 2

stack.yaml

version: "3.3"

services:
  caddy:
    image: caddy:2-alpine
    hostname: "{{.Service.Name}}.{{.Task.Slot}}"
    ports:
      - 80:80
      - 443:443
      - 8080:8080
    # entrypoint: ["sh", "-c", "while true; do echo sleep; sleep 1; done"]
    deploy:
      replicas: 2
      restart_policy:
        condition: on-failure

  mongo:
    image: mongo:4.4-bionic
    hostname: "{{.Service.Name}}.{{.Task.Slot}}"