feat: add experimental docker collector from STATE_DB

[rokernel]

Summary

• add a new experimental docker_collector that reads SONiC STATE_DB DOCKER_STATS|* entries with background cache refresh and default disabled (DOCKER_ENABLED=false)
• export per-container CPU, memory, network, block IO, and PID metrics with a single container label, plus collector/source health metrics (source_stale, entries_skipped, cache/scrape duration)
• wire collector registration in main, add fixture coverage and unit tests, and document config + usage in README.md

Safety and behavior

• read-only implementation (Redis reads only, no Docker socket access, no command execution)
• cap exported series with DOCKER_MAX_CONTAINERS
• track source freshness with DOCKER_SOURCE_STALE_THRESHOLD
• tolerate LastUpdateTime field/timestamp format differences across SONiC versions

Validation

• go test ./...
• go build ./...
• runtime checks on two lab SONiC switches succeeded:
  • sonic_docker_collector_success reported healthy
  • sonic_docker_containers exported non-zero values
  • sonic_docker_entries_skipped stayed at expected baseline
  • sonic_docker_source_stale indicated fresh source data