docs: Add comprehensive DG_MAX_RATIO tuning guide

Created extensive documentation for the DG_MAX_RATIO parameter, which
controls delta compression efficiency thresholds.

New Documentation:
- docs/DG_MAX_RATIO.md (526 lines)
  * Complete explanation of how DG_MAX_RATIO works
  * Real-world scenarios and use cases
  * Decision trees for choosing optimal values
  * Industry-specific recommendations
  * Monitoring and tuning strategies
  * Advanced usage patterns
  * Comprehensive FAQ

Updates to Existing Documentation:
- README.md: Added link to DG_MAX_RATIO guide with tip callout
- CLAUDE.md: Added detailed DG_MAX_RATIO explanation and guide link
- Dockerfile: Added inline comments explaining DG_MAX_RATIO tuning
- docs/sdk/getting-started.md: Added DG_MAX_RATIO guide reference

Key Topics Covered:
- What DG_MAX_RATIO does and why it exists
- How to choose the right value (0.2-0.7 range)
- Real-world scenarios (nightly builds, major versions, etc.)
- Industry-specific use cases (SaaS, mobile apps, backups, etc.)
- Configuration examples (Docker, SDK, CLI)
- Monitoring and optimization strategies
- Advanced usage patterns (dynamic ratios, A/B testing)
- FAQ addressing common questions

Examples Included:
- Conservative (0.2-0.3): For dissimilar files or expensive storage
- Default (0.5): Balanced approach for most use cases
- Permissive (0.6-0.7): For very similar files or cheap storage

Value Proposition:
- Helps users optimize compression for their specific use case
- Prevents inefficient delta compression
- Provides data-driven tuning methodology
- Reduces support questions about compression behavior

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Dockerfile

@@ -71,6 +71,11 @@ HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
 ENV DG_LOG_LEVEL=INFO
 
 # Performance & Compression
+# DG_MAX_RATIO: Maximum delta/file ratio (0.0-1.0)
+# Default 0.5 means: only use delta if delta_size ≤ 50% of original_size
+# Lower (0.2-0.3) = more conservative, only high-quality compression
+# Higher (0.6-0.7) = more permissive, accept modest savings
+# See docs/DG_MAX_RATIO.md for complete tuning guide
 ENV DG_MAX_RATIO=0.5
 
 # Cache Configuration