ci: Use base image from Alpine container registry

Docker does not support loongarch64 for official images yet. Alpine
publishes images for all architectures on their registry, so just use
that instead.

See also https://gitlab.alpinelinux.org/alpine/docker-abuild/-/merge_requests/71

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7738>

.ci/build-jobs.yaml.j2

@@ -1,4 +1,4 @@
-image: alpine:latest
+image: registry.alpinelinux.org/img/alpine:latest
 stages:
   - build
   - hardware tests