How do I copy variables between stages of multi stage Docker build?

Question

I've only seen examples of using COPY to copy files between stages of a multi stage Dockerfile, but is there a way to simply copy an ENV variable? My use case is to start out with a git image to just to get the commit hash that will be part of the build. The image I'm later building with hasn't got git.

I realise I could just pipe out the git hash to a file and use COPY but I'm just wondering if there's a cleaner way?

Answer 1

You got 3 options: The "ARG" solution, the "base"-stage solution, and "file" solution.

"ARG" solution

ARG version_default=v1

FROM alpine:latest as base1
ARG version_default
ENV version=$version_default
RUN echo ${version}
RUN echo ${version_default}

FROM alpine:latest as base2
ARG version_default
RUN echo ${version_default}

"base"-stage solution

another way is to use base container for multiple stages:

FROM alpine:latest as base
ARG version_default
ENV version=$version_default

FROM base
RUN echo ${version}

FROM base
RUN echo ${version}

You can find more details here: https://github.com/moby/moby/issues/37345

"file" solution

Save the hash into a file in the first stage, copy it in the second stage, and then read it and use it there.