For some reason amd64-GitCommit didn't error before (unless I'm mistaken and missed this...), but now it appears amd64-* is just implied. Drop amd64-GitCommit and use vanilla GitCommit.
@@ -8,5 +8,5 @@ GitRepo: https://github.com/archlinux/archlinux-docker.git
Tags: latest, 20191006
-amd64-GitCommit: 671e950b6f879a346fffa2f9fbaaba8ace07efae
+GitCommit: 671e950b6f879a346fffa2f9fbaaba8ace07efae
GitFetch: refs/tags/20191006