How to add user properly in docker in nixes, in order to fix CASA's Fluid-Attacks-Result warning?

Question

docker file:

FROM nixos/nix
# https://stackoverflow.com/questions/27701930/how-to-add-users-to-docker-container
# RUN useradd -ms /bin/bash newuser
# USER newuser
RUN useradd -m matthew
USER matthew
WORKDIR /usr/scan
COPY . /usr/scan/
RUN mkdir results
RUN nix-env -if https://github.com/fluidattacks/makes/archive/22.09.tar.gz