This account is not available: Unable to switch user in python alpine docker container

I have a simple Dockerfile

FROM python:3.10-alpine# Set environment variablesENV PYTHONDONTWRITEBYTECODE 1ENV PYTHONUNBUFFERED 1# Set argumentsARG USERNAME=jpgARG USER_DIR=/home/$USERNAMEARG WORK_DIR=$USER_DIR/app# Creating a non-root userRUN adduser -S $USERNAME# Switching the userUSER $USERNAME# Create a new directory to keep the project filesRUN mkdir $WORK_DIR# Copy local contents into containerCOPY ./ $WORK_DIR# Setting newly created directory as PWDWORKDIR $WORK_DIR# Adding user's bin path to `PATH` variableENV PATH "$PATH:$USER_DIR/.local/bin"# Installing pip packagesRUN pip install pip -U

I built the image using following command

docker build . -t test-img:latest

After the successfull image build, run the image using

docker run -i -t test-img:latest python

This gave me an interactive shell, as expected. Also, I tried to pull the user info using getuser(...) function as well

Python 3.10.10 (main, Mar 14 2023, 03:01:12) [GCC 12.2.1 20220924] on linuxType "help", "copyright", "credits" or "license" for more information.>>> import getpass>>> getpass.getuser()'jpg'>>> 

Then, I moved to another terminal and logged in to the same container as root user. After logged-in, tried to switch the user to jpg (which is defined within the Dockerfile)

$ docker exec -it -u root f6dc515bca29 sh                                                                                                                                                          /home/jpg/app # su - jpgThis account is not available/home/jpg/app # su - "jpg"This account is not available/home/jpg/app # su -l "jpg"This account is not available/home/jpg/app # su -l jpgThis account is not available/home/jpg/app # 

Question

How can I switch the user to jpg (or any non-root user) in an alpine-based container from root?

Note: It is mandatory to log in as root (-u root) in an exec session because this is to simulate a situation of AWS Fargate containers.