Automated periodic checks for stale base images.

[willfurnass]

Is your feature request related to a problem? Please describe.

Base image tags referenced in Dockerfiles in this repo are quite old and relate to images with lots of associated serious CVEs.

Describe the solution you'd like

GitHub Actions workflow to periodically build Docker images from this repo, scan with a vuln scanner such as Trivy (but other options exist) then alert if high-severity unfixed vulns are found. These alerts could potentially autogenerate public Issues or private security alerts to maintainers.

Describe alternatives you've considered
This scanning could also potentially be handled by an image registry.