Reapply "fedora-image-uploader: Add staging config for container uploads"
This reverts commit 15dbcbb7ac01d738b58603c58627fc219f97cd60, which was
a revert of commit 5e0ad1134d7ed7796f8385d6960edfc1ef16d31a (pr #2200).
Adam Williamson got rid of the need for buildah so hopefully we won't
run into lots of permission issues.