Add PULSE_DISABLE_DOCKER_UPDATE_CHECKS environment variable and
--disable-docker-update-checks flag to disable Docker image update
detection. This is useful for:
- Avoiding Docker Hub rate limits
- Users who don't want update notifications in their dashboard
Related to Discussion #982
- Add GHCR (GitHub Container Registry) token support for public images
- Clean up dockerUpdateFirstSeen tracking when containers are removed
- Improve UpdateIcon tooltip to show digest info
- Add cursor-help to indicate hoverable tooltip