feat: add verbose option for detailed cache logging

[adityamaru]

Summary

• Adds a new verbose input parameter to the action for enhanced debugging capabilities
• Implements detailed BuildKit cache logging using docker buildx du instead of basic filesystem commands
• Provides visibility into cache usage patterns and pruning effectiveness
• Prepares code structure for new metrics (BPA_V2_DEBUG_WORKERS_AVAILABLE_MS and BPA_V2_PRUNE_BYTES)

Changes

• action.yml: Added verbose boolean input (default: false)
• pruneBuildkitCache: Updated to accept verbose parameter and use docker buildx du for detailed cache information
• Cache logging: When verbose is enabled, logs show:
  • Cache size for each layer
  • Last used timestamps
  • Cache types (regular, source, exec.cachemount, etc.)
  • Descriptions of cache contents
  • Total cache size before and after pruning
• Future metrics: Added commented code structure for:
  • BPA_V2_DEBUG_WORKERS_AVAILABLE_MS: Track time for workers to become available
  • BPA_V2_PRUNE_BYTES: Track bytes pruned from cache
  • These will be enabled when updating to @buf/blacksmith_vm-agent.bufbuild_es@2.7.0+

Test plan

• Unit tests updated and passing
• Build successful
• Test with verbose: false (default) - should not show cache details
• Test with verbose: true - should show detailed cache information
• Verify cache pruning still works correctly

Usage Example

- uses: useblacksmith/setup-docker-builder@main
  with:
    verbose: true  # Enable detailed cache logging

Future Work

When the buf packages are updated to v2.7.0+, uncomment the metric tracking code to automatically report:

• Worker availability timing metrics
• Cache pruning size metrics

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