ROOT base image base image

The Issue title isn't a typo. I just want to know at the start of the Dockerfile that will be used to build the root-base image, what is X in

ARG BASE_IMAGE=X
FROM ${BASE_IMAGE} as base

At the moment I'm using python:3.7-slim but this brings up the question on if it is okay to use a Debian base image, or do we need CentOS instead?