ARG is for setting environment variables which are used during the docker build process – they are not present in the final image, which is why you don’t see them when you use docker run.
You use ARG for settings that are only relevant when the image is being built, and aren’t needed by containers which you run from the image. You can use ENV for environment variables to use during the build and in containers.
With this Dockerfile:
FROM ubuntu
ARG BUILD_TIME=abc
ENV RUN_TIME=123
RUN touch /env.txt
RUN printenv > /env.txt
You can override the build arg as you have done with docker build -t temp --build-arg BUILD_TIME=def .. Then you get what you expect:
> docker run temp cat /env.txt
HOSTNAME=b18b9cafe0e0
RUN_TIME=123
HOME=/root
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
BUILD_TIME=def
PWD=/