ubuntu - Cannot create directory. Permission denied inside docker container

Question

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

FROM ubuntu:16.04

RUN apt-get update && 
    apt-get -y install sudo

RUN adduser --disabled-password --gecos '' newuser 
    && adduser newuser sudo 
    && echo '%sudo ALL=(ALL:ALL) ALL' >> /etc/sudoers

USER newuser

RUN mkdir -p /newfolder
WORKDIR /newfolder

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

Answer 1

Filesystems inside a Docker container work just like filesytems outside a Docker container: you need appropriate permissions if you are going to create files or directories. In this case, you're trying to create /newfolder as a non-root user (because the USER directive changes the UID used to run any commands that follow it). That won't work because / is owned by root and has mode dr-xr-xr-x.

Try instead:

RUN mkdir -p /newfolder
RUN chown newuser /newfolder
USER newuser
WORKDIR /newfolder

This will create the directory as root, and then chown it.