dockerfile - How do I set environment variables during the build in docker

Question

I'm trying to set environment variables in docker container during the build but without success. Setting them when using run command works but I need to set them during the build.

Dockerfile

FROM ubuntu:latest
ARG TEST_ENV=something

Command I'm using to build

docker build -t --build-arg TEST_ENV="test" myimage .

Running

docker run -dit myimage

I'm checking available environment variables by using

docker exec containerid printenv

and the result is

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=e49c1abfd58b
TERM=xterm
no_proxy=*.local, 169.254/16
HOME=/root

TEST_ENV is not present

Answer 1

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=/