Non parallel multiarch

Possibly more successful multiarch pipeline run with max-parallelism=1 - https://gitlab.developers.cam.ac.uk/uis/devops/infra/dockerimages/-/jobs/2226216

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/admin/-/issues/incident/156

build-and-push.gitlab-ci.yml

@@ -9,10 +9,7 @@
     - apk update
     - apk add bash gettext  # for envsubst command
     - apk add jq # for querying manifests
-    # The official docs state that `docker buildx create --use` should be enough on its own. However, there appears to be
-    # a bug which, even though this issue is closed https://github.com/docker/buildx/issues/584#issuecomment-830041059,
-    # still requires this `docker run --rm` workaround.
-    - docker run --rm --privileged multiarch/qemu-user-static --reset -p yes || true; docker buildx create --use
+    - ./prep-buildx.sh
   script:
     - ./build.sh
   variables: