Recommended base image
As suggested in my comment, you could write a Dockerfile that looks like:
FROM python:3

RUN pip install --no-cache-dir --upgrade pip && \
    pip install --no-cache-dir nibabel pydicom matplotlib pillow med2image
    # Note: we had to merge the two "pip install" package lists here, otherwise
    # the last "pip install" command in the OP may break dependency resolution…

CMD ["cat", "/etc/os-release"]

And the command example above could confirm at runtime (docker build --pull -t test . && docker run --rm -it test) that this image is based on the GNU/Linux distribution "Debian stable".
Generic Dockerfile template
Finally to give a comprehensive answer, note that a good practice regarding Python dependencies consists in specifying them in a declarative way in a dedicated text file (in alphabetical order, to ease review and update) so that for your example, you may want to write the following file:

requirements.txt

matplotlib
med2image
nibabel
pillow
pydicom


and use the following generic Dockerfile

FROM python:3

WORKDIR /usr/src/app

COPY requirements.txt ./

RUN pip install --no-cache-dir --upgrade pip \
  && pip install --no-cache-dir -r requirements.txt

COPY . .

CMD ["python", "./your-daemon-or-script.py"]

To be more precise, this is the approach suggested in the documentation of the Docker official image python, §. How to use this image