Passing argument to dockerfile not working well - docker

Question

I'm having some trouble with passing argument value to dockerfile.
Running docker in Windows Server 2016
My docker version info is
PS C:\Users\Administrator\Desktop> docker version
Client: Docker Engine - Enterprise
Version: 19.03.4
API version: 1.40
Go version: go1.12.10
Git commit: 9e27c76fe0
Built: 10/17/2019 23:42:50
OS/Arch: windows/amd64
Experimental: false
Server: Docker Engine - Enterprise
Engine:
Version: 19.03.4
API version: 1.40 (minimum version 1.24)
Go version: go1.12.10
Git commit: 9e27c76fe0
Built: 10/17/2019 23:41:23
OS/Arch: windows/amd64
Experimental: false
and powershell opened as administrator.
My reference is this.
But it not worked for me.
So this is my dockerfile.
FROM microsoft/iis
ARG a_version
RUN echo $a_version
Also i tried many different types of echo value
Such as
RUN echo "$a_version"
RUN echo ${a_version}
RUN echo "${a_version}"
And this is my execute command.
docker build . --build-arg a_version=1234
My expected result was print 1234
But actual result was
PS C:\Users\Administrator\Desktop> docker build . --build-arg a_version=1234
Sending build context to Docker daemon 14.04MB
Step 1/3 : FROM microsoft/iis
---> 595015675977
Step 2/3 : ARG a_version
---> Running in 91698d9e71da
Removing intermediate container 91698d9e71da
---> 2bad94a2ce74
Step 3/3 : RUN echo $a_version
---> Running in 3fe25ecb813c
$a_version
Why it happens? How can is fix it?

Answer 1

In Windows Command-Prompt the syntax is echo %a_version% as your
base image is based on window.
how-can-i-display-the-contents-of-an-environment-variable-from-the-command-promp
So you can change this to
FROM microsoft/iis
ARG a_version
RUN echo %a_version%
env in window dockerfile

Answer 2

I had to do something simmilar and this is a piece of code that worked for me.
ARG PORT_SITE
ENV PORT_SITE ${PORT_SITE}
and then I refer it in the entrypoint as following
$env:PORT_SITE
That's the way I did it, but i suppose that you can ommit the env variable and just refer to the arg value using
${PORT_SITE}