Don't use the full registry url in FROM on Tumbleweed

This is discouraged in TW: https://build.opensuse.org/request/show/1202139

src/bci_build/package/__init__.py

@@ -834,10 +834,14 @@ def dockerfile_from_line(self) -> str:
             return ""
 
         if self.from_target_image:
-            return (
-                f"FROM {self.base_image_registry}/{self.from_target_image} AS target\n"
-                f"FROM {self._from_image} AS builder"
-            )
+            # use the full registry URL in FROM on SLE,
+            # but on TW we must not do this
+            if self.os_version.is_tumbleweed:
+                target = self.from_target_image
+            else:
+                target = f"{self.base_image_registry}/{self.from_target_image}"
+
+            return f"FROM {target} AS target\nFROM {self._from_image} AS builder"
 
         return f"FROM {self._from_image}"