feat: Add Docker healthcheck support

[austinc3030]

Summary

This PR adds a built-in healthcheck command to the cloudflare-ddns binary, enabling Docker health monitoring for DDNS update containers.

Changes

Code Changes

• Added --healthcheck flag to the main binary in cmd/ddns/ddns.go
• Implemented healthcheckMain() function that:
  • Validates configuration
  • Detects current public IP address(es)
  • Attempts to update DNS records
  • Returns exit code 0 on success, 1 on failure

Documentation Changes

• Added comprehensive "Docker Health Monitoring" section to README
• Included Docker Compose healthcheck example
• Documented healthcheck configuration options
• Added best practices for healthcheck timing and IPv6 considerations
• Provided monitoring commands and usage examples

Dockerfile Changes

• Added whitespace formatting for better readability

Why This Feature is Useful

Docker healthchecks provide automated monitoring of container health, which is crucial for production deployments:

1. Automatic Failure Detection: Detect when DDNS updates fail without manual monitoring
2. Container Orchestration: Enable automatic restarts or failover in Docker Swarm/Kubernetes
3. Monitoring Integration: Works with standard Docker health status monitoring tools
4. Real-world Validation: Tests the actual DDNS update process, not just if the binary is running

Usage Example

healthcheck:
  test: ["CMD", "/bin/ddns", "--healthcheck"]
  interval: 5m
  timeout: 30s
  start_period: 30s
  retries: 3

Testing

• Verified the --healthcheck flag correctly validates configuration
• Tested successful healthcheck with valid credentials and IP detection
• Confirmed proper error reporting when configuration or updates fail
• Validated exit codes (0 for success, 1 for failure)

Important Notes

⚠️ Users should set IP6_PROVIDER=none if their Docker environment doesn't support IPv6, as the healthcheck will fail if either IPv4 or IPv6 detection fails when both are enabled.

[Sc0rc3d]

I would Love healthcheck in this Image!