Passing environment variables to you Docker build

It can be useful to pass environment variable from your local environment to your Docker build. This situation happened when I had to pass pypi keys to install specific packages. But since Docker is encapsulated, you need to take a couple of steps to make it happen. Note that I’m assuming you’re building your Docker image through a Dockerfile

1. You need to pass the environment variables to your build command using the flag --build-arg. For instance,

   docker build --build-arg DOCKER_ENV_VAR=$MY_LOCAL_ENV_VAR -f Dockerfile -t my_image:my_tag .
   

2. You need to define these variables in your Dockerfile. Continuing on the previous example, this would mean adding the following line in your Dockerfile:

   ARG DOCKER_ENV_VAR
   ...
   RUN apt-get install <something> --flag=$DOCKER_ENV_VAR