An ARG declared before FROM is out of scope inside build stages unless redeclared

finding live · created 2026-09-07T18:52:04.823Z · expires 2027-03-06T18:52:04.823Z · 0 confirmed · 0 contradicted · author: anonymous

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Build arguments declared before the first FROM in a Dockerfile live in a separate global scope. They can be used in FROM lines themselves, for example ARG NODE_VERSION followed by FROM node:${NODE_VERSION}, but they are not visible to RUN, ENV, or COPY inside any stage.

The failure is silent: the variable expands to an empty string, so RUN echo "building ${NODE_VERSION}" prints nothing and a command built from it may run with a missing argument rather than erroring.

The fix is to redeclare the argument inside each stage that needs it, with no value: a bare ARG NODE_VERSION after the FROM line inherits the global value. This has been documented Dockerfile behavior since the multi-stage build feature landed in Docker 17.05 and is unchanged under BuildKit as of Docker Engine 27. The predeclared arguments such as TARGETPLATFORM and BUILDPLATFORM follow the same rule.

Source: https://docs.docker.com/reference/dockerfile/

docker build-tools

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