How Do I Set Environment Variables During the Build in Docker
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...
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
2 Answers
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=/
I'll summarize highlights from this great article.
ARG Vs ENV
1 ) Short explanation:
ARG is only available during the build of a Docker image (RUN etc), not after the image is created and containers are started from it (ENTRYPOINT, CMD).
ENV values are available to containers, but also RUN-style commands during the Docker build starting with the line where they are introduced. If you set an environment variable in an intermediate container using bash (RUN export VARI=5 && …) it will not persist in the next command.
2 ) Longer explanation:
ARG are also known as build-time variables. They are only available from the moment they are ‘announced’ in the Dockerfile with an ARG instruction up to the moment when the image is built. Running containers can’t access values of ARG variables. This also applies to CMD and ENTRYPOINT instructions which just tell what the container should run by default. If you tell a Dockerfile to expect various ARG variables (without a default value) but none are provided when running the build command, there will be an error message.
However, ARG values can be easily inspected after an image is built, by viewing the docker history of an image. Thus they are a poor choice for sensitive data.
ENV variables are also available during the build, as soon as you introduce them with an ENV instruction. However, unlike ARG, they are also accessible by containers started from the final image. ENV values can be overridden when starting a container, more on that below.
3 ) With diagrem:
Here is a simplified overview of ARG and ENV availabilities around the process around building a Docker image from a Dockerfile, and running a container.
They overlap, but ARG is not usable from inside the containers.
Robert Thorne covers electric vehicle innovations, autonomous driving systems, global mobility trends, and automotive engineering developments.
