Add XDG Base Directory support for configuration file lookup #2569

Hi! I'd like to propose adding support for the XDG Base Directory specification as a secondary configuration location.
Current Behavior:
DefaultDockerClientConfig looks for configuration in ~/.docker/config.json (and other Docker-standard locations).
Proposed Behavior:
Add ~/.config/docker-java/configuration.properties as an additional lookup location, maintaining full backward compatibility.

Rationale:

Many users prefer XDG conventions to keep ~/ clean
Hundreds of config files in home directory becomes difficult to manage
This would be additive only - existing paths continue to work
Read-only nature means low risk

Implementation Approach:

Primary: Check existing locations (current behavior unchanged)
Secondary: Check ~/.config/docker-java/configuration.properties if primary not found
Fallback to defaults as currently done