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Do not refer to an undefined variable.
Superseded by buildkit/UndefinedVar, which provides the same check with typo detection.

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Description

Ensures that environment variables and build arguments are declared before being used. Undeclared variables can lead to unexpected behavior or build errors.

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Examples

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Problematic code

FROM alpine
COPY $foo .

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Correct code

FROM alpine
ARG foo
COPY $foo .

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Reference