Set default $USER as root

[hikaruna]

$ docker run -it --rm rust:1.45.0-buster cargo new app
error: Failed to create package `app` at `/app`

Caused by:
  could not determine the current user, please set $USER

To solve this

$ docker run -it --rm -e USER=root rust:1.45.0-buster cargo new app

It's very annoying to have to do this. Docker user is of course root in normal use, so I want you to set it as the default value.