feat(ci): support private Docker registry via env vars

Add DOCKER_REGISTRY and DOCKER_REGISTRY_PREFIX environment variables
to ci/build_matrix.py for pulling base images from a private registry
(Artifactory, Harbor, etc.) instead of Docker Hub.

Document CI usage and registry configuration in README.

ci/build_matrix.py

@@ -2,6 +2,7 @@
 """Parse meta/main.yml platforms and output a Docker test matrix as JSON."""
 
 import json
+import os
 import sys
 from pathlib import Path
 
@@ -18,6 +19,14 @@ PLATFORM_IMAGE_MAP = {
     "Ubuntu": "ubuntu",
 }
 
+def _resolve_image(image: str) -> str:
+    """Prefix image with registry and namespace if configured via env vars."""
+    registry = os.environ.get("DOCKER_REGISTRY", "").strip().rstrip("/")
+    prefix = os.environ.get("DOCKER_REGISTRY_PREFIX", "").strip()
+    if registry:
+        return f"{registry}/{prefix}{image}"
+    return image
+
 def build_matrix(meta_path: str = "roles/remote_users_fact/meta/main.yml") -> list[dict]:
     meta_file = Path(meta_path)
     if not meta_file.exists():
@@ -43,9 +52,10 @@ def build_matrix(meta_path: str = "roles/remote_users_fact/meta/main.yml") -> li
         for version in platform.get("versions", []):
             version_str = str(version)
             slug = f"{name.lower()}{version_str}" if name == "EL" else f"{name.lower()}-{version_str}"
+            base_image = f"{image_prefix}:{version_str}"
             matrix.append({
                 "slug": slug,
-                "image": f"{image_prefix}:{version_str}",
+                "image": _resolve_image(base_image),
                 "platform": name,
                 "version": version_str,
             })