Add image source to docker file

[cutoffthetop]

PR Context

• demo PR for the release pipeline changes: feature/mx-1509 update container build and push mex-backend#84
• demo container index https://github.com/cutoffthetop/mex-backend/pkgs/container/mex-backend
• demo release page https://github.com/cutoffthetop/mex-backend/releases

Added

• Add more metadata labels to docker file
• Add hint to PR template to explain testing for template changes
• Configure sensible concurrency rules for all workflows
• Add distribution upload step to release pipeline

Changes

• Trigger documentation on release pipeline instead of pushed tag
• Harmonize cron schedules

Removed

• Remove docker workflow and move into release pipeline
• Remove stale dependabot cve-scan config

Fixed

• Fix tagging and pushing docker containers to ghcr.io