TARGETARCH/TARGETPLATFORM scope and behaviour

[droopy4096]

It seems that scope for "automatic" TARGETARCH and TARGETPLATFORM are no well defined. Consider example:

FROM alpine
ARG TARGETPLATFORM
RUN echo "I'm building for $TARGETPLATFORM" > /var/tmp/platform

as per docs - it works as expected

however

FROM alpine
RUN echo "I'm building for $TARGETPLATFORM" > /var/tmp/platform

does not - producing "empty" output

Yet, when I use it within FROM ... statement it seems to work

FROM --platform=${TARGETPLATFORM} alpine
RUN uname -m > /var/tmp/platform

to take it further I've tried

FROM --platform="" alpine
RUN uname -m > /var/tmp/platform

and it works just fine with default cli options. So I ended up using:

docker buildx build --platform=linux/amd64 -f Dockerfile -t my-my .

with previous Dockerfile and that did result in expected:

0.193 standard_init_linux.go:219: exec user process caused: exec format error

in other words confirming that automatic vars like $TARGETPLATFORM have "odd" scoping:

• they are available immediately for FROM ... statements
• they are not available for each stage execution unless explicitly requested via ARG.

❓ Question:

Moving forward should we always use ARG TARGETARCH and such to declare intent to use that automatic variable or is this an intended behaviour and FROM ... will receive automatic variable without declaration yet anywhere else there needs to be explicit declaration?