Pin Docker-cpu `FROM ubuntu:20.04` (#7677)

utils/docker/Dockerfile-cpu

 # YOLOv5 🚀 by Ultralytics, GPL-3.0 license
 
 # Start FROM Ubuntu image https://hub.docker.com/_/ubuntu
-FROM ubuntu:latest
+FROM ubuntu:20.04
 
 # Install linux packages
 RUN apt update
 RUN DEBIAN_FRONTEND=noninteractive TZ=Etc/UTC apt install -y tzdata
-RUN apt install -y python3-pip git zip curl htop screen libgl1-mesa-glx libglib2.0-0 software-properties-common
-
-# Install python3.9
-RUN add-apt-repository ppa:deadsnakes/ppa -y
-RUN apt install python3.9 python3.9-distutils libpython3.9 -y
-# RUN alias python=python3.9
+RUN apt install -y python3-pip git zip curl htop screen libgl1-mesa-glx libglib2.0-0
+# RUN alias python=python3
 
 # Install pip packages
 COPY requirements.txt .
-RUN python3.9 -m pip install --upgrade pip
-RUN python3.9 -m pip install --no-cache -r requirements.txt albumentations gsutil notebook \
+RUN python3 -m pip install --upgrade pip
+RUN pip install --no-cache -r requirements.txt albumentations gsutil notebook \
     coremltools onnx onnx-simplifier onnxruntime openvino-dev tensorflow-cpu tensorflowjs \
     torch torchvision --extra-index-url https://download.pytorch.org/whl/cpu