Changes the intermediate image building steps to use registry caching, allowig us to always rebuild them, but do so very quickly when nothing has changed

.github/scripts/common.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python3
+
+
+def get_image_tag(
+    repo_name: str,
+    pkg_name: str,
+    pkg_version: str,
+) -> str:
+    """
+    Returns a string representing the normal image for a given package
+    """
+    return f"ghcr.io/{repo_name}/builder/{pkg_name}:{pkg_version}"
+
+
+def get_cache_image_tag(
+    repo_name: str,
+    pkg_name: str,
+    pkg_version: str,
+    branch_name: str,
+) -> str:
+    """
+    Returns a string representing the expected image cache tag for a given package
+
+    Registry type caching is utilized for the builder images, to allow fast
+    rebuilds, generally almost instant for the same version
+    """
+    return f"ghcr.io/{repo_name}/builder/cache/{pkg_name}:{pkg_version}"