How to export an environment variable to a docker image?

Question

I can define "static" environment variables in a Dockerfile with ENV, but is it possible to pass some value at build time to this variable? I'm attempting something like this, which doesn't work:

FROM phusion/baseimage

RUN mkdir -p /foo/2016/bin && \
    FOOPATH=`ls -d /foo/20*/bin` && \
    export FOOPATH

ENV PATH $PATH:$FOOPATH

Of course, in the real use case I'd be running/unpacking something that creates a directory whose name will change with different versions, dates, etc., and I'd like to avoid modifying the Dockerfile every time the directory name changes.

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Edit: Since it appears it's not possible, the best workaround so far is using a symlink:

FROM phusion/baseimage

RUN mkdir -p /foo/2016/bin && \
    FOOPATH=`ls -d /foo/20*/bin` && \
    ln -s $FOOPATH /mypath

ENV PATH $PATH:/mypath

Answer 1

To pass a value in at build time, use an ARG.

FROM phusion/baseimage

RUN mkdir -p /foo/2016/bin && \
    FOOPATH=`ls -d /foo/20*/bin` && \
    export FOOPATH

ARG FOOPATH
ENV PATH $PATH:${FOOPATH}

Then you can run docker build --build-arg FOOPATH=/dir -t myimage .

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Edit: from you comment, my answer above won't solve your issue. There's nothing in the Dockerfile you can update from the output of the run command, the output isn't parsed, only the resulting filesystem is saved. For this, I think you're best off in your run command writing the path to the image and read in from your /etc/profile or a custom entrypoint script. That depends on how you want to launch your container and the base image.