Adding a user in a linux docker container

Dockerfile

FROM yourimage

# Setup the user
ENV USER_NAME=yourusername
ENV GROUP_NAME=yourusergroup # can be the same as USER_NAME
ENV HOME=/var/home/$USER_NAME

# Create the home directory for the new user.
RUN mkdir -p $HOME

# Create a user so our program doesn't run as root.
# For debian based linux
RUN groupadd -r $GROUP_NAME &&\
    useradd -r -g $GROUP_NAME -d $HOME -s /sbin/nologin -c "Docker image user" $USER_NAME
    
# For alpine based linux
# ENV USER_ID=65535
# ENV GROUP_ID=65535
# RUN addgroup -g $USER_ID $GROUP_NAME && \
#    adduser --shell /sbin/nologin --disabled-password \
#    --no-create-home --uid $USER_ID --ingroup $GROUP_NAME $USER_NAME

## SETTING UP THE APP ##
WORKDIR $HOME

# (Optional) Copy in the application code.
# ADD . $HOME

# Chown all the files to the user.
RUN chown -R $USER_NAME:$GROUP_NAME $HOME

# Change to the user.
USER $USER_NAME