Q:

Using ARG and ENV in Dockerfile

I am learning how to use ARG and ENV in Dockerfiles.
I have this simple Dockerfile:
ARG my_arg
ARG other_arg=other_default

FROM centos:7

ENV MY_ENV $my_arg
ENV OTHER_ENV $other_arg

CMD echo "$MY_ENV $OTHER_ENV"

When I build it:
docker build --build-arg my_arg=my_value
And run it:
docker run <resulting-image>
I do not see the expected output, which would be 
my_value other_default

Instead, I see the empty string.
What am I doing wrong?

A:

In a Dockerfile, each FROM line starts a new image, and generally resets the build environment.  If your image needs to specify ARGs, they need to come after the FROM line; if it's a multi-stage build, they need to be repeated in each image as required.  ARG before the first FROM are only useful to allow variables in the FROM line, but can't be used otherwise.
This is further discussed under Understand how ARG and FROM interact in the Dockerfile documentation.
FROM centos:7

# _After_ the FROM line
ARG my_arg
ARG other_arg=other_default
...

