Workaround a behaviour of s3cmd when the source local path is `-`
s3cmd uses the - filename to indicate that it should check stdin for
input. https://github.com/s3tools/s3cmd/blob/b4f1424a18a3231230e6c5676d88cd68aa24feba/S3/S3.py#L633
We use the - directory in gitlab uploads as a place to store system
appearance uplods, like a custom logo/favicon. We use a directory that
isn't a valid gitlab user/group name so we don't have to have reserved
user/groups.