Prevent deletion of container images required for startup

[zoetrope]

What

When our data center is shut down for more than 72 hours, the container images required for startup may be deleted.

• https://github.com/cybozu-go/neco/blob/cf1fbc99aeb62d37e37c5c2c69f12e319622b6cd/debian/lib/systemd/system/docker-prune.service
• https://github.com/cybozu-go/neco/blob/cf1fbc99aeb62d37e37c5c2c69f12e319622b6cd/ignitions/common/systemd/docker-prune.service

How

Describe how to address the issue.

Checklist

• [ ] Finish implementation of the issue
• [ ] Test all functions
• [ ] Have enough logs to trace activities
• [ ] Notify developers of necessary actions