Heroku docker deployment
I had this problem when I forgot to run heroku container:login. Even if you're logged in to heroku, heroku container service requires a separate login. That's why you're getting the error right at the beginning of the upload - you are not authorised.
I had the same error when I forgot to heroku container:login first. Not sure if your problem was caused by the same user error.
registry.heroku.com/mediabox/web
Just to create an app with name 'mediabox' by using of heroku dashboard and try again to push image, it works for me.