Extends the Docker monitoring and alerting system to track writable layer
usage as a percentage of the container's root filesystem. This helps
identify containers with bloated copy-on-write layers before they
consume excessive disk space.
- Add disk threshold to DockerThresholdConfig (default: 85% trigger, 80% clear)
- Evaluate disk alerts for running containers when RootFilesystemBytes > 0
- Include disk metadata (writable layer, total filesystem, block I/O stats)
- Update frontend to display and configure disk thresholds
- Add test coverage for disk usage alert hysteresis
- Document disk monitoring in DOCKER_MONITORING.md
Per-container and per-host overrides apply to disk thresholds the same
way they do for CPU and memory.
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