Can't run visudo within Docker; sudoers file does not exist

Question

Ubuntu n00b here. I am tinkering with a Docker container that I created using the following simple Dockerfile:

FROM ubuntu
CMD ["tail","-f","/dev/null"]

Inside the container, as the root user, I try running visudo and get the following response:

bash: visudo: command not found

I ran ls /etc and it seems I don't have a sudoers file either.

Is there something special I should have put in my Dockerfile to make these exist?

Answer 1

I worked it out from another answer: sudo is not installed by default. To install sudo:

apt-get update
apt-get install sudo

Answer 2

When you built the container, you are root so you can create users without sudo.

If you are deriving a container image that already has a USER directive, you can still become root again with USER root before using privileged commands. You can later issue another USER directive to become the execution user again. In other words:

An image that sets a non privilege user:

# "plainuser" image
FROM debian
RUN groupadd -g 1000 appgroup && useradd -g appgroup -u 1000 appuser
USER appuser
ENTRYPOINT ["/usr/bin/id"]

Run with docker run --rm plainuser to check that it indeed runs with user appuser.

Let's derive it:

# "otheruser" image
FROM plainuser
USER root  # Become root again
RUN groupadd -g 1001 appgroup2 && useradd -g appgroup2 -u 1001 appuser2
USER appuser2 # Set new execution user

Run with docker run --rm otheruser.