Following from this, I am trying to install like so (in docker, but also same error directly on host):

RUN apt-get update && \
    apt-get install -y software-properties-common && \
    add-apt-repository -y ppa:deadsnakes/ppa && \
    apt-get update && \
    apt install -y python3.7

I am getting

E: Unable to locate package python3.7
E: Couldn't find any package by glob 'python3.7'
E: Couldn't find any package by regex 'python3.7'

This was working, and one day it stopped. I don't know if I did something, or if something happened on deadsnakes or elsewhere.

How to get Python3.7 in the docker, and in the host?