Setup Python3.8 in Ubuntu Xenial (16.04 LTS). The python version was fixed at Python 3.5.1

setup-python3.8-via-ppa.sh

#!/bin/sh

# install PPA
sudo add-apt-repository ppa:deadsnakes/ppa

# update and install
sudo apt update
sudo apt install python3.8 python3.8-dev python3.8-venv

# setup alternatives
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.5 1
sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 2

# show menu for selecting the version
sudo update-alternatives --config python3

# or one command to set it
# sudo update-alternatives --set python3 /usr/bin/python3.8

it doesn't work any more

duplicate this issue with a Dockerfile

$ cat Dockerfile

FROM ubuntu:xenial

RUN apt update
RUN apt install -y software-properties-common
RUN add-apt-repository ppa:deadsnakes/ppa -y
RUN apt update
RUN apt install -y python3.8

build the image

$ docker build -t demo . 

...
 > [6/6] RUN apt install -y python3.8:
#9 0.222
#9 0.222 WARNING: apt does not have a stable CLI interface. Use with caution in scripts.
#9 0.222
#9 0.224 Reading package lists...
#9 0.960 Building dependency tree...
#9 1.098 Reading state information...
#9 1.148 E: Unable to locate package python3.8
#9 1.148 E: Couldn't find any package by glob 'python3.8'
#9 1.148 E: Couldn't find any package by regex 'python3.8'

update

seems deadsnakes team doesn't support ubuntu xenial (16.04) any more.

ref: https://launchpad.net/~deadsnakes/+archive/ubuntu/ppa

I gave up, have to switch to Ubuntu Bionic or Focal.