What I want to achieve

I have a project that I want to be able to build both with docker build (for compatibility) and docker buildx --platform and which requires TARGETPLATFORM to make a difference in the build process. As TARGETPLATFORM is not set by docker build, the default value option to ARG seemed the right choice.

What I expect

Given the following Dockerfile,

FROM debian:buster-slim
ARG TARGETPLATFORM=linux/amd64
RUN echo $TARGETPLATFORM; exit 1

I expect docker build . on all architectures to output linux/amd64 (the legacy compatibility case).

When running docker buildx --builder docker-multiarch --platform linux/386 ., the output should be linux/386.

What I get

On platform linux/amd64, docker buildx --builder docker-multiarch --platform linux/386 ., outputs linux/amd64 instead of linux/386.

Removing the default value solves this: docker buildx --builder docker-multiarch --platform linux/386 . results in linux/386 with Dockerfile:

FROM debian:buster-slim
ARG TARGETPLATFORM
RUN echo $TARGETPLATFORM; exit 1

However, docker build . results in an empty/undefined TARGETPLATFORM.

(For ARGs set with --build-arg on the command line, defaults behave as expected.)

System setup

• Ubuntu 20.10
• Docker 19.03.13
• Buildx 0.4.1 or 0.5.1 (tested with both)

Workaround

The following Dockerfile results in the expected behavior:

FROM debian:buster-slim
ARG TARGETPLATFORM
ENV TARGETPLATFORM=${TARGETPLATFORM:-linux/amd64}
RUN echo $TARGETPLATFORM; exit 1

However, this (a) defeats the purpose of the default value and (b) causes endless debugging sessions, as it violates the documentation of ARG and TARGETPLATFORM.