Answer by aaron for How to include files outside of Docker's build context?

I often find myself utilizing the --build-arg option for this purpose. For example after putting the following in the Dockerfile:

ARG SSH_KEYRUN echo "$SSH_KEY"> /root/.ssh/id_rsa

You can just do:

docker build -t some-app --build-arg SSH_KEY="$(cat ~/file/outside/build/context/id_rsa)" .

But note the following warning from the Docker documentation:

Warning: It is not recommended to use build-time variables for passing secrets like github keys, user credentials etc. Build-time variable values are visible to any user of the image with the docker history command.