How to specify a file for environment variables fo

Question

I have a few environment variables that I need inside of my code.

And when I starts building a docker container, I wish to bake those environment variables into my docker container.

I understand that the dockerfile directive 'ENV' does exactly that, I wonder if there is a way to point to an file instead of writing it inside of the Dockerfile

Answer 1

At build time, ENV variables are not available. You need to use ARG variables. For that you have the "--build-arg" flag of "docker build". It doesn't take in a file thou.

ENV variables are used by a running container and can be injected as a file using the "--env-file" flag of "docker run".

Hopefully this is what you were looking for otherwise let me know more in detail.

Answer 2

AFAIK, there is no such way to inject environment variables using a file during the build step using Dockerfile. However, in most cases, people do end up using an entrypoint script & injecting variables during the docker run or docker-compose up.

In case it's a necessity you might need to write a shell wrapper which will change the values in the Dockerfile dynamically by taking a key-value pair text file as an input or make it something as below but the ENV file name need to be included in Dockerfile as suggested by @thaJeztah in link -

COPY my-env-vars /    
RUN export $(cat my-env-vars | xargs)

It's an open issue - https://github.com/moby/moby/issues/28617