Docs: fix alpine example due to python version

[cbrwflo]

Fixes the Alpine example, which removed the generic python package in v3.12.

node-gyp supports python3 since v5.x: https://github.com/nodejs/node-gyp/blob/master/CHANGELOG.md

[iradul]

Thank you for the PR. Could you also update the image from FROM node:8-alpine to FROM node:14-alpine?

[cbrwflo]

Sorry for the delay -- tested and this builds fine.