docs: Comprehensive environment variable documentation

Added complete documentation for all environment variables across
Dockerfile, README.md, and SDK documentation.

Dockerfile Changes:
- Documented all DeltaGlider environment variables with defaults
- Added AWS configuration variables (commented for runtime override)
- Updated version label to 5.0.3
- Updated description to mention encryption

README.md Changes:
- Added comprehensive Docker Usage section
- Documented all environment variables with examples
- Added Docker examples for:
  * Basic usage with AWS credentials
  * Memory cache configuration for CI/CD
  * MinIO/custom endpoint usage
  * Persistent encryption key setup
- Security notes for encryption and cache behavior

SDK Documentation Changes:
- Added DeltaGlider Configuration section
- Documented all environment variables
- Added configuration examples
- Security notes for encryption behavior

Environment Variables Documented:
- DG_LOG_LEVEL (logging configuration)
- DG_MAX_RATIO (compression threshold)
- DG_CACHE_BACKEND (filesystem or memory)
- DG_CACHE_MEMORY_SIZE_MB (memory cache size)
- DG_CACHE_ENCRYPTION_KEY (optional persistent key)
- AWS_ENDPOINT_URL (custom S3 endpoints)
- AWS_ACCESS_KEY_ID (AWS credentials)
- AWS_SECRET_ACCESS_KEY (AWS credentials)
- AWS_DEFAULT_REGION (AWS region)

Quality Checks:
- All 119 tests passing ✅
- Type checking: 0 errors (mypy) ✅
- Linting: All checks passed (ruff) ✅
- Dockerfile syntax validated ✅

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

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

Dockerfile

@@ -66,10 +66,28 @@ USER deltaglider
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
     CMD deltaglider --help || exit 1
 
+# Environment variables (all optional, can be overridden at runtime)
+# Logging
+ENV DG_LOG_LEVEL=INFO
+
+# Performance & Compression
+ENV DG_MAX_RATIO=0.5
+
+# Cache Configuration
+ENV DG_CACHE_BACKEND=filesystem
+ENV DG_CACHE_MEMORY_SIZE_MB=100
+# ENV DG_CACHE_ENCRYPTION_KEY=<base64-key>  # Optional: Set for cross-process cache sharing
+
+# AWS Configuration (override at runtime)
+# ENV AWS_ENDPOINT_URL=https://s3.amazonaws.com
+# ENV AWS_ACCESS_KEY_ID=<your-key>
+# ENV AWS_SECRET_ACCESS_KEY=<your-secret>
+# ENV AWS_DEFAULT_REGION=us-east-1
+
 # Labels
 LABEL org.opencontainers.image.title="DeltaGlider" \
-      org.opencontainers.image.description="Delta-aware S3 file storage wrapper" \
-      org.opencontainers.image.version="0.1.0" \
+      org.opencontainers.image.description="Delta-aware S3 file storage wrapper with encryption" \
+      org.opencontainers.image.version="5.0.3" \
       org.opencontainers.image.authors="Beshu Limited" \
       org.opencontainers.image.source="https://github.com/beshu-tech/deltaglider"