ci: Tag multiarch images on GHCR too

Author: def-

misc/python/materialize/mzbuild.py

@@ -1471,6 +1471,19 @@ def publish_multiarch_images(
                 ]
             )
             spawn.runv(["docker", "manifest", "push", name])
+
+            ghcr_prefix = "ghcr.io/materializeinc"
+            ghcr_name = f"{ghcr_prefix}/{name}"
+            spawn.runv(
+                [
+                    "docker",
+                    "manifest",
+                    "create",
+                    ghcr_name,
+                    *(f"{ghcr_prefix}/{image.spec()}" for image in images),
+                ]
+            )
+            spawn.runv(["docker", "manifest", "push", ghcr_name])
     print(f"--- Nofifying for tag {tag}")
     markdown = f"""Pushed images with Docker tag `{tag}`"""
     spawn.runv(