adduser -D is ambiguous 

I am getting the following error for the given Dockerfile:
`Option d is ambiguous (debug, disabled-login, disabled-password)`

https://github.com/heroku/alpinehelloworld/blob/40073bae3b3e103b60043226ea183e2821a2c8aa/Dockerfile#L19

It should be `RUN adduser --disabled-login myuser` (or --disabled-password)