Sudo not available in docker image despite being installed in base image - docker

Question

I can't access Sudo from my container, despite it looking like the base image has it installed.
nistmni#ca5af2f4aace:~$ sudo echo x
bash: sudo: command not found
nistmni#ca5af2f4aace:~$ /bin/sudo
bash: /bin/sudo: No such file or directory
My Dockerfile is simple:
FROM nistmni/minc-toolkit
RUN mkdir ~/execute
COPY . ~/execute/
CMD /bin/bash
The Dockerfile for nistmni/minc-toolkit is:
FROM ubuntu:xenial
# install basic system packages
RUN apt-get -y update && \
apt-get -y dist-upgrade && \
apt-get install -y --no-install-recommends \
sudo \
build-essential g++ gfortran bc \
bison flex \
libx11-dev x11proto-core-dev \
libxi6 libxi-dev \
libxmu6 libxmu-dev libxmu-headers \
libgl1-mesa-dev libglu1-mesa-dev \
libjpeg-dev \
libssl-dev ccache libapt-inst2.0 git lsb-release \
curl ca-certificates unzip && \
apt-get autoclean && \
rm -rf /var/lib/apt/lists/*
# add user to build all tools
RUN useradd -ms /bin/bash nistmni && \
echo "nistmni ALL=(ALL) NOPASSWD: ALL" > /etc/sudoers.d/nistmni && \
chmod 0440 /etc/sudoers.d/nistmni
ENV PATH=/usr/lib/ccache:$PATH
# add new cmake
RUN mkdir src && \
cd src && \
curl -L --output cmake-3.14.5.tar.gz https://github.com/Kitware/CMake/releases/download/v3.14.5/cmake-3.14.5.tar.gz && \
tar zxf cmake-3.14.5.tar.gz && \
cd cmake-3.14.5 && \
./configure --prefix=/usr --no-qt-gui && \
make && \
make install && \
cd ../../ && \
rm -rf src
USER nistmni
ENV HOME /home/nistmni
WORKDIR /home/nistmni
Is this:
Some subtlety of base images I don't understand
Sudo is getting removed somewhere I'm not seeing
Not actually the Dockerfile being used to create the image
Thanks.