About

The compose file references the image docker.internal:5000/monit:latest that does not exist publicly because the image was built using the provided monit Dockerfile.

The /conf mount within the guest should be mapped to a folder on the host and it will be read by monit on startup. Ideally, a small settings file should be present with the following contents:

set daemon 120            # check services at 2-minute intervals
set logfile /dev/stdout
set httpd port 2812
    use address 0.0.0.0
    allow admin:swordfish
include services/*

that:

sets monit on a two minute timer,
the log file is redirected to stdout such that it can be observable from docker,
the HTTP access port is set to 2812 and the HTTP server listens on all addresses (necessary for Docker) with the username admin and password swordfish,
finally, monit is instructed to load all the service files from the services/ subfolder from the current directory where the user can place monit services to monitor.

Source

version: '3.8'
services:
  monit:
    image: docker.internal:5000/monit:latest
    ports:
      - 10001:2812
    volumes:
      - /mnt/docker/data/monit:/conf
    deploy:
      labels:
        - shepherd.enable=true
        - shepherd.auth.config=local
      replicas: 3
      placement:
        max_replicas_per_node: 1
      resources:
        limits:
          cpus: "0.50"
          memory: 200M