Can we pass ENV variables through cmd line while building a docker image through dockerfile?

Question

I am working on a task that involves building a docker image with centOs as its base using a Dockerfile . One of the steps inside the dockerfile needs http_proxy and https_proxy ENV variables to be set in order to work behind the proxy.

As this Dockerfile will be used by multiple teams having different proxies, I want to avoid having to edit the Dockerfile for each team. Instead I am looking for a solution which allows me to pass ENV variables at build time, e.g.,

sudo docker build -e http_proxy=somevalue .

I'm not sure if there is already an option that provides this. Am I missing something?

Answer 1

Containers can be built using build arguments (in Docker 1.9+) which work like environment variables.

Here is the method:

FROM php:7.0-fpm ARG APP_ENV=local ENV APP_ENV ${APP_ENV} RUN cd /usr/local/etc/php && ln -sf php.ini-${APP_ENV} php.ini 

and then build a production container:

docker build --build-arg APP_ENV=prod .

For your particular problem:

FROM debian ENV http_proxy ${http_proxy} 

and then run:

docker build --build-arg http_proxy=10.11.24.31 .

Note that if you build your containers with docker-compose, you can specify these build-args in the docker-compose.yml file, but not on the command-line. However, you can use variable substitution in the docker-compose.yml file, which uses environment variables.

Answer 2

So I had to hunt this down by trial and error as many people explain that you can pass ARG -> ENV but it doesn't always work as it highly matters whether the ARG is defined before or after the FROM tag.

The below example should explain this clearly. My main problem originally was that all of my ARGS were defined prior to FROM which resulted all the ENV to be undefined always.

# ARGS PRIOR TO FROM TAG ARE AVAIL ONLY TO FROM for dynamic a FROM tag ARG NODE_VERSION FROM node:${NODE_VERSION}-alpine  # ARGS POST FROM can bond/link args to env to make the containers environment dynamic ARG NPM_AUTH_TOKEN ARG EMAIL ARG NPM_REPO  ENV NPM_AUTH_TOKEN ${NPM_AUTH_TOKEN} ENV EMAIL ${EMAIL} ENV NPM_REPO ${NPM_REPO}  # for good measure, what do we really have RUN echo NPM_AUTH_TOKEN: $NPM_AUTH_TOKEN && \   echo EMAIL: $EMAIL && \   echo NPM_REPO: $NPM_REPO && \   echo $HI_5 # remember to change HI_5 every build to break `docker build`'s cache if you want to debug the stdout  ..... # rest of whatever you want RUN, CMD, ENTRYPOINT etc..