How to write text to a file?

Question

Docker

I'm trying to create my own image based on the ubuntu:latest image, in which the /helloworld file will exist with the name of the current system user. Here is the Dockerfile:

FROM ubuntu
COPY helloworld /helloworld
RUN echo $USER > /helloworld

After creating the image, I run it as a container (I hope I'm not confusing anything in terminology) in interactive mode:
docker run -t -i docktor888/hw /bin/bash
and I plan to see the username: in this file
ls && cat /helloworld
, but this file is empty.
Please help me fix the Dockerfile so that /helloworld contains the name of the current user.
PS:
Of course, before starting the build, I manually create
touch /helloworld

Answer 1

Most likely, the $USER variable is simply not defined at the build stage.
Even worse, you can redefine the user when starting the container from the image, and such a docker image will become meaningless:

docker run -u <айди пользователя> <имя образа> whoami

decision? Write docker-entrypoint.sh (specify it as entrypoint), which will create this file during container startup.