Distribution: build-arg not set during build

[Duoxilian]

Take this simple dockerfile:

ARG IMAGE
ARG FILES
FROM ${IMAGE}
RUN echo ${IMAGE} > /usr/local/src/image
RUN echo ${FILES} > /usr/local/src/files

I run a docker build as follows:

$ docker build . --tag test:latest --file dockerfile --build-arg IMAGE=ubuntu:latest --build-arg FILES=files

When I spin up a container with this image, the two files created are empty:

$ docker run -d -t -p 8081:8081 test /bin/bash
$ docker exec -it focused_mestorf bash
root@45189afac13a:/# cd /usr/local/src/
root@45189afac13a:/usr/local/src# ls
files  image
root@45189afac13a:/usr/local/src# cat *


root@45189afac13a:/usr/local/src# 

This used to work in the past (docker.io). Has anything changed about build-arg handling?

[Duoxilian]

I am sorry I neglected to state my operating environment: Ubuntu 17.04, 64-bit, Intel® Core™ i7-7700HQ CPU @ 2.80GHz × 8, 15.5 GB.

[thaJeztah]

In a Dockerfile, FROM starts a new build-stage, and any property from the previous stage (if any) is reset. In your example, the Dockerfile should likely be like this:

ARG IMAGE
FROM ${IMAGE}
ARG IMAGE
ARG FILES
RUN echo ${IMAGE} > /usr/local/src/image
RUN echo ${FILES} > /usr/local/src/files

In other words: to be able to use the IMAGE and/or FILES build-arg, they have to be defined within the stage (so _after_ the FROM).

There have been some bug fixes in recent Docker releases related to properties from previous stages not being properly reset, so possibly this previously worked due to a bug that has now been fixed; you mentioned that it worked in the docker.io packages; not that those are not the official Docker packages, so it's possible there's still a bug in those.

Also, be aware that this repository (and issue tracker) is for the open source Docker _registry_, not Docker itself; if you discover a bug in Docker, it's best to report this in the Docker issue tracker, located at https://github.com/docker/for-linux/issues.

I'm closing this issue, because this doesn't look like a bug, but feel free to continue the conversation

[Duoxilian]

Thanks very much for your guidance. Works just like you said.

I'll check whether this is described anywhere in the manuals.