Add structured bug-fix exercise guide and surface it in README

[Copilot]

This PR turns the request into a practical debugging curriculum: it documents a curated set of real bugs in the codebase as exercises, with reproducible symptoms and clear completion criteria. The goal is to let contributors practice realistic, work-style debugging without pre-fixing the issues.

• Exercise catalog added

  • Introduced BUG_EXERCISES.md with 9 targeted bug scenarios spanning:
    • runtime panics and edge cases
    • file I/O correctness
    • process/state consistency
    • integration contract mismatches
    • resource-leak patterns
    • config/path consistency
    • extraction security boundaries
    • error-propagation gaps
    • shared mutable state/concurrency risk
  • Each exercise includes: category, exact file location, symptom, repro hint, debugging focus, and “done when” criteria.

• Discoverability in core docs

  • Added a Debugging Exercises entry in README.md linking directly to BUG_EXERCISES.md so new contributors can start from the main doc path.

• Repo hygiene for local builds

  • Updated .gitignore to exclude basic-docker-engine binary output to prevent accidental commits of local artifacts.

Example exercise format now included in the repo:

## Exercise 5 — Deferred close in loop can leak descriptors under many layers
- **Category:** Resource leak / scalability bug
- **Location:** `image.go` (`Pull`)
- **Symptom:** Layer readers are deferred inside a loop, delaying close until function return.
- **Done when:** Each layer reader is closed promptly after extraction.