You can try using conda. I used several stages to minimize final container and to speedup/cache local builds.

# first stage
FROM nvidia/cuda:11.1-base-ubuntu18.04 as builder
RUN apt-get update && apt-get install -y curl wget gcc build-essential

# install conda
RUN wget --quiet https://repo.anaconda.com/miniconda/Miniconda3-4.5.12-Linux-x86_64.sh -O ~/miniconda.sh && \
     /bin/bash ~/miniconda.sh -b -p /opt/conda

# create env with python 3.5
RUN /opt/conda/bin/conda create -y -n myenv python=3.5
    
# install requirements
WORKDIR /app
COPY requirements.txt /app
ENV PATH=/opt/conda/envs/myenv/bin:$PATH    
RUN pip install -r requirements.txt
RUN pip uninstall -y pip


####################
# second stage (note: FROM container must be the same as builder)
FROM nvidia/cuda:11.1-base-ubuntu18.04 as runner

# copy environment data including python
COPY --from=builder /opt/conda/envs/myenv/bin /opt/conda/envs/myenv/bin
COPY --from=builder /opt/conda/envs/myenv/lib /opt/conda/envs/myenv/lib
# do some env settings
ENV PATH=/opt/conda/envs/myenv/bin:$PATH
ENV LC_ALL=C.UTF-8
ENV LANG=C.UTF-8


####################
# final image
from runner
WORKDIR /app    
COPY ./run.py /app
CMD [ "python", "run.py"]

If I look at the Dockerfile here, it installs python3 which by default is installing python 3.5 for debian:stretch. You can instead install python 3.7 by editing the Dockerfile and building it yourself. In your Dockerfile, remove lines 19-25 and replace line 1 with the following, and then build the image locally.

FROM python:3.7-stretch

If you are not familiar with building your own image, download the Dockerfile and keep it in its own standalone directory. Then after cd into the directory, run the command below . You may want to first remove the already downloaded image. After this you should be able to run other docker commands the same way as if you had pulled the image from docker hub.

docker build -t gettyimages/spark .

I don't think you can find a prebuilt python3.9 package on a debian 8 distribution as your environment is pretty old.

The only solution is you build the python3.9 out from source code in your base container. A full workable Dockerfile as next:

FROM node:9-slim

RUN apt update; \
apt install -y build-essential zlib1g-dev libncurses5-dev libgdbm-dev libnss3-dev libssl-dev libreadline-dev libffi-dev; \
wget https://www.python.org/ftp/python/3.9.7/Python-3.9.7.tgz; \
tar -zxvf Python-3.9.7.tgz; \
cd Python-3.9.7; \
./configure --prefix=/usr/local/python3; \
make && make install; \
ln -sf /usr/local/python3/bin/python3.9 /usr/bin/python3; \
ln -sf /usr/local/python3/bin/pip3.9 /usr/bin/pip3

Verify it:

$ docker build -t myimage:1 .
$ docker run --rm -it myimage:1 python3 --version
Python 3.9.7
$ docker run --rm -it myimage:1 pip3 --version
pip 21.2.3 from /usr/local/python3/lib/python3.9/site-packages/pip (python 3.9)

Unless you explicitly in Dockerfile upgrade python, the python version in image is 3.7 no doubt, at least in the Dockerfile you afford I didn't see this:

$ docker run --rm -it python:3.7 python --version
Python 3.7.12

Additional, a possible reason maybe you sometimes in the past, make a wrong tag for python:3.7 with python:3.10, like next, then it could result your issues:

$ docker tag python:3.10 python:3.7
$ docker run --rm -it python:3.7 python --version
Python 3.10.0

If above is your case, do next to clean the environment, then repeat your steps to check it again:

$ docker rmi python:3.7