Cannot create directory. Permission denied inside docker container

Can not create folder during image building with non root user added to sudoers group. Here is my Dockerfile:

FROM ubuntu:16.04RUN apt-get update && \    apt-get -y install sudoRUN adduser --disabled-password --gecos '' newuser \&& adduser newuser sudo \&& echo '%sudo ALL=(ALL:ALL) ALL'>> /etc/sudoersUSER newuserRUN mkdir -p /newfolderWORKDIR /newfolder

I get error: mkdir: cannot create directory '/newfolder': Permission denied