python:3 base image points to wrong version

[lurch]

As discovered by @garethtdavies at balena-io/docs#1285 (comment)

You'd expect balenalib/raspberrypi4-64-alpine-python:3-3.11-run to be using the newest version of Python3 (i.e. 3.8.0) but for some reason it's using Python 3.6.9 😕
I guessed at a possible reason here

[lurch]

Looks like the version of Python included in a python:3 base image is indeed semi-random 😕
balenalib/raspberrypi4-64-alpine-python:3-3.11-run now includes Python 3.7.5 (whereas balena-io/docs#1322 says the latest version of Python 3.7.x is 3.7.6 and the latest version of Python 3.x is 3.8.1)

[nghiant2710]

@lurch The new versions in the docs above are not available yet (still being built).

About the wrong version, yeah we do have a bug in contrato - the engine that produces base images combinations, each 3.x series will have a 3-* tag and they will replace each other depend on which is the last tag built by Jenkins. I will fix it soon and in the meantime, I think it's better if you use the tag that has major and minor version e.g. 3.7-* or full version then you will have an image with correct python version as described in the docs

[lurch]

This isn't actually a feature I need myself, but when this bug is fixed I was going to suggest to @garethtdavies that it might be worth mentioning in the docs that python (without a version tag) will get the latest version of Python 2, but that python:3 can be used to get the latest version of Python 3 (since Python 2 is now officially EOL).
😃