Passing build args in multi-stage build using a terminal stage dockerfile

[rverma-nikiai]

This is a feature suggestion, which I believe would be much useful.

Consider scenario as there is a util (e.g brew) which can install multiple packages

FROM alpine:3.7 as build

ONBUILD ARG CLOUD
ONBUILD RUN if [ "${CLOUD}" = "aws" ]; then echo 'installing aws'; fi
ONBUILD RUN if [ "${CLOUD}" = "gce" ]; then echo 'installing gce'; fi

As a consumer of this utility, I can install the utilities from the base image, while adding multiple utilities of mine also;

FROM my-onbuild
RUN echo 'installing my-super-cool-aws-util'

But at final step I would need to build my image as
docker build --build-arg CLOUD=aws -t cool .

It would be nice to have if from final Image (cool here) I define those args within docker file,
and they are automatically added to build context's args even for builder image

FROM my-onbuild
ARG CLOUD=aws
RUN echo 'installing my-super-cool-aws-util'

Raising it in cli repo as 1 quick way to handle this is, scan the current docker file add all non-null args as build-args (if not already added).

Thanks

[thaJeztah]

Haven't tried (writing from my phone), but have you tried if it works if you define the default on a global level? ie;

ARG CLOUD=aws
FROM my-onbuild
RUN echo 'installing my-super-cool-aws-util'

[rverma-nikiai]

@thaJeztah It works, thanks a lot.

[thaJeztah]

Great!

[DaWoody]

Hi @thaJeztah I was hoping you could help me answer a quick question about passing variables between multi-staged builds in a Dockerfile, without using global arguments. To make it more clear of why I am asking this.

My use-case (snippet from a fictive Dockerfile) is something like this:

FROM alpine AS pre_install
# extract node-version from file here, pseudo-code something like
NODE_VERSION=$(echo $(cat .nvmrc))
# Now I want to build the second build stage where the real stuff happens
FROM node-alpine:${NODE_VERSION} AS build_app
# .. do stuff here..

... The problem is ofc how its possible to pass the NODE_VERSION variable into the second build-stage, hence I prior to this do not yet have that information. I cannot use a global ARG NODE_VERSION before the first FROM, here since the information is extracted from the content within the build folder (Where the Dockerfile resides)

Ofc I can do stuff prior to this using bash/shell and script stuff... but that is not at all as clean as making it work directly in Docker. It should be possible to make it work without resorting to external scripts and then calling docker build --build-arg=NODE_VERSION -t cool-app-name:1.0.0 .

Any idea on how can I do this ?

Thanks a lot in advance 🌞!
Cheers!

[thaJeztah]

You can't set Dockerfile variables from the result of a shell command (and no, a local ARG cannot be used to set a global arg).

[DaWoody]

@thaJeztah ok Thanks for the answer! Which is: its not possible from within a Dockerfile to generate a value that will later be used in latter multistage FROM section ( the pattern according to the code snippet provided). I must know outside of the Dockerfile what version I am to use in all or any FROM sections in a single or multistaged build Dockerfile?