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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) $FOO will print an empty string.
So if all of the above is correct, my question is if there is a way for the docker build context to inherit the environment of the shell being called from.
407
ENV values are accessible during the build, and afterwards once the container runs. You can set ENV values in your Dockerfile - either by hardcoding them, or in a dynamic fashion.
Using docker-compose , you can inherit env variables in docker-compose. yml and subsequently any Dockerfile(s) called by docker-compose to build images. This is useful when the Dockerfile RUN command should execute commands specific to the environment.
You can use the ARG command inside a Dockerfile to define the name of a parameter and its default value. This default value can also be overridden using a simple option with the Docker build command.
You could define default values with ARG:
ARG build_var=default_value
ENV ENV_VAR=$build_var
and then override at build time:
docker build --build-arg build_var=$HOST_VAR
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