Dockerfile: Inherit environmental variables from shell

Question

When building an image using a Dockerfile, in order to make some env vars available to the docker build context, one should explicitly declare associations of the form

ENV MYVAR=MYVALUE

AFAIK (correct me if I am misguided here), the environmental variables exported to the shell from which the docker build command is executed, are not passed to the Docker build context, i.e. if in my shell I have previously

export FOO=BAR

having the following declaration in my Dockerfile

ENV FOO=$FOO

and then echoing (still from within the Dockerfile) $FOO will print an empty string.

So if all of the above is correct, my question is if there is a way for the docker build context to inherit the environment of the shell being called from.

Answer 1

You could define default values with ARG:

ARG build_var=default_value
ENV ENV_VAR=$build_var

and then override at build time:

docker build --build-arg build_var=$HOST_VAR