Where you could not see it? in a Docker container using default Docker images for several OSes including Debian family. But you don’t usually check man there but in the outside OS in which you edit the source code containing that README in order to prepare your custom Docker container or image with it.

That was there. I recognized this in Docker.

Howerver, in Docker, Alpine is not the only nor the main provider of images nor almost the main used.

There are quite a lot based in Debian as I use it even as base for custom images and download images using it.