Docker environment variable not available in RUN command - docker

Question

I am having difficulties with ARG & ENV in docker after I have upgraded to Docker version 20.10.7, build f0df350 on windows 10.
I have made dockerfile to show issue:
FROM microsoft/dotnet:2.2-aspnetcore-runtime AS base
ARG node_build=production
ENV node_build_env=${node_build}
FROM node:12.18.3 AS node-build
WORKDIR /root
RUN echo $node_build_env > test.txt
FROM base AS final
WORKDIR /app
COPY --from=node-build /root/test.txt ./
My goal here is that an ARG can be set and it will be then set as environment variable inside the container and if none is set it has a default value.
In this Dockerfile I am attempting to write the environment variable node_build_env to a text file then copy it to the final layer. The problem though is that the file is empty.
To re-create these are commands I am using:
docker build -t testargs:test .
docker run -it --rm testargs:test /bin/bash
cat test.txt
The file is empty. However if I run:
docker build -t testargs:test . --target node-build
and then manually run the command:
echo $node_build_env > test.txt
It works and the value production is written into the file.
Why does it work when I do it manually but not as part of the RUN command?

Answer 1

You are using multi-stage builds.
Your ARG & ENV belongs to base stage. And you're not using your base stage in your node-build build stage.
That means there is no node_build_env value in node-build. Hence the following line creates an empty test.txt file.
RUN echo $node_build_env > test.txt
However your final stage uses base stage. Which means it has access to node_build_env variable. So after building your image using docker build -t testargs:test . and then open up an interactive session with that container and try to execute the following command,
echo $node_build_env
You will see production will be printed out in the terminal.
I believe this will help you solve the problem. Cheers 🍻 !!!
edit:
this is working version:
ARG node_build=production
FROM microsoft/dotnet:2.2-aspnetcore-runtime AS base
FROM node:12.18.3 AS node-build
ARG node_build
ENV node_build_env=$node_build
WORKDIR /root
RUN echo $node_build_env > test.txt
FROM base AS final
WORKDIR /app
COPY --from=node-build /root/test.txt ./