Selfhosted

Sure, it's a bit hack-and-slash, but not too bad. Honestly the dockcheck portion is already pretty complete, I'm not sure what all you could add to improve it. The custom plugin I'm using does nothing more than dump the array of container names with available updates to a comma-separated list in a file. In addition to that I also have a wrapper for dockcheck which does two things:

1. dockcheck plugins only run when there's at least one container with available updates, so the wrapper is used to handle cases when there are no available updates.
2. Some containers aren't handled by dockcheck because they use their own management system, two examples are bitwarden and mailcow. The wrapper script can be modified as needed to support handling those as well, but that has to be one-off since there's no general-purpose way to handle checking for updates on containers that insist on doing things in their own custom way.

Basically there are 5 steps to the setup:

1. Enable Prometheus metrics from Docker (this is just needed to get running/stopped counts, if those aren't needed it can skipped). To do that, add the following to /etc/docker/daemon.json (create it if necessary) and restart Docker:

{
  "metrics-addr": "127.0.0.1:9323"
}

Once running, you should be able to run curl http://localhost:9323/metrics and see a dump of Prometheus metrics

2. Clone dockcheck, and create a custom plugin for it at dockcheck/notify.sh:

send_notification() {
Updates=("$@")
UpdToString=$(printf ", %s" "${Updates[@]}")
UpdToString=${UpdToString:2}

File=updatelist_local.txt

echo -n $UpdToString > $File
}

3. Create a wrapper for dockcheck:

#!/bin/bash

cd $(dirname $0)

./dockcheck/dockcheck.sh -mni

if [[ -f updatelist_local.txt ]]; then
  mv updatelist_local.txt updatelist.txt
else
  echo -n "None" > updatelist.txt
fi

At this point you should be able to run your script, and at the end you'll have the file "updatelist.txt" which will either contain a comma-separated list of all containers with available updates, or "None" if there are none. Add this script into cron to run on whatever cadence you want, I use 4 hours.

4. The main Python script:

#!/usr/bin/python3

from flask import Flask, jsonify

import os
import time
import requests
import json

app = Flask(__name__)

# Listen addresses for docker metrics
dockerurls = ['http://127.0.0.1:9323/metrics']

# Other dockerstats servers
staturls = []

# File containing list of pending updates
updatefile = '/path/to/updatelist.txt'

@app.route('/metrics', methods=['GET'])
def get_tasks():
  running = 0
  stopped = 0
  updates = ""

  for url in dockerurls:
      response = requests.get(url)

      if (response.status_code == 200):
        for line in response.text.split("\n"):
          if 'engine_daemon_container_states_containers{state="running"}' in line:
            running += int(line.split()[1])
          if 'engine_daemon_container_states_containers{state="paused"}' in line:
            stopped += int(line.split()[1])
          if 'engine_daemon_container_states_containers{state="stopped"}' in line:
            stopped += int(line.split()[1])

  for url in staturls:
      response = requests.get(url)

      if (response.status_code == 200):
        apidata = response.json()
        running += int(apidata['results']['running'])
        stopped += int(apidata['results']['stopped'])
        if (apidata['results']['updates'] != "None"):
          updates += ", " + apidata['results']['updates']

  if (os.path.isfile(updatefile)):
    st = os.stat(updatefile)
    age = (time.time() - st.st_mtime)
    if (age < 86400):
      f = open(updatefile, "r")
      temp = f.readline()
      if (temp != "None"):
        updates += ", " + temp
    else:
      updates += ", Error"
  else:
    updates += ", Error"

  if not updates:
    updates = "None"
  else:
    updates = updates[2:]

  status = {
    'running': running,
    'stopped': stopped,
    'updates': updates
  }
  return jsonify({'results': status})

if __name__ == '__main__':
  app.run(host='0.0.0.0')

The neat thing about this program is it's nestable, meaning if you run steps 1-4 independently on all of your Docker servers (assuming you have more than one), then you can pick one of the machines to be the "master" and update the "staturls" variable to point to the other ones, allowing it to collect all of the data from other copies of itself into its own output. If the output of this program will only need to be accessed from localhost, you can change the host variable in app.run to 127.0.0.1 to lock it down. Once this is running, you should be able to run curl http://localhost:5000/metrics and see the running and stopped container counts and available updates for the current machine and any other machines you've added into "staturls". You can then turn this program into a service or launch it @reboot in cron or in /etc/rc.local, whatever fits with your management style to start it up on boot. Note that it does verify the age of the updatelist.txt file before using it, if it's more than a day old it likely means something is wrong with the dockcheck wrapper script or similar, and rather than using the output the REST API will print "Error" to let you know something is wrong.

5. Finally, the Homepage custom API to pull the data into the dashboard:

        widget:
          type: customapi
          url: http://localhost:5000/metrics
          refreshInterval: 2000
          display: list
          mappings:
            - field:
                results: running
              label: Running
              format: number
            - field:
                results: stopped
              label: Stopped
              format: number
            - field:
                results: updates
              label: Updates

Thanks so much for the reply, I'll give it a download to play with it - I certainly am a big fan of simple!