Why my Dockerfile build-arg is not working as ENV - dockerfile

Question

I have this Dockerfile:
ARG VER=INVALID
FROM python:${VER}
ENV VERSION ${VER}
CMD ["/bin/bash", "-c", "echo VERSION = $VERSION"]
To build it I use:
sudo docker build --tag teste --build-arg VER=3 .
And when I run it, I'm getting:
$ sudo docker run teste
VERSION =
If I run it using the export command, I'm getting the VERSION ENV empty:
$ sudo docker run teste /bin/bash -c export
...[other ENVs]...
declare -x PYTHON_VERSION="3.9.7"
declare -x VERSION=""
But the VER build-arg seems to working for the image base version (FROM python:${VER}).
Why? How can I solve this?

Answer 1

According the docker DOC:
An ARG declared before a FROM is outside of a build stage, so it can’t be used in any instruction after a FROM. To use the default value of an ARG declared before the first FROM use an ARG instruction without a value inside of a build stage:
Source: https://docs.docker.com/engine/reference/builder/#understand-how-arg-and-from-interact
So, I solved redeclaring the ARG after the FROM, like this:
ARG VER=INVALID
FROM python:${VER}
ARG VER
ENV VERSION ${VER}
CMD ["/bin/bash", "-c", "echo VERSION = $VERSION"]