Another few months have passed and thanks to a of user contributions and suggestions a bunch of changes got implemented, big and small.
The two latest changes have been pretty large:

• Complete rewrite of notification logics
  • Configuration is set through the dockcheck.config
  • Templates used “untouched”
  • Possibility to trigger multiple notification templates through “channels”
• Restructure the update process
  • First pulls all (selected) images
  • Then recreate all containers that received updates - to avoid unnecessary restarts and strain

https://github.com/mag37/dockcheck

Plenty more changes have been implemented since I posted last, such as:

• Added a config-file to set user options (same as passing option flags).
• Added option -u for unattended dockcheck self update (caution!).
• Added option -I to print urls from url.list to list of containers with updates.
• Cleaned up and refactored a lot of code;
  • Safer variables and pipefail options.
  • Consistent colorization of messages.
  • Monochrome mode hides progress bar.
  • Exits if pull or recreation of container fails.
• Cleared up some readme with extra info;
  • Synology DSM
  • Prometheus + node_exporter
  • Zabbix config
  • Rest API script
  • Unraid wrapper script
• Permission checks;
  • Graceful exit if no docker permissions.
  • pkg-manager installs handles sudo/doas/root properly.
• Notify-templates; added slack, added markdown support to some templates.

I’m very happy to have a supportive and contributing user base who helps with troubleshooting, suggesting changes and contributing code. Thank you!