The variable is not available in the current scope. It will be evaluated to an empty value.
Why is this an issue?
The variables defined by ARG instruction have a scope from the definition to the end of the build stage where it was defined. If it
was defined in the beginning of the Dockerfile (outside of any build stage), then its scope is restricted to only FROM instructions.
Outside of their scope, variables will be resolved to empty string which may lead to unintended behaviour.
How to fix it
Code examples
Noncompliant code example
ARG SETTINGS
FROM busybox
RUN ./run/setup $SETTINGS
In this case the $SETTINGS variable will be evaluated to empty string.
Compliant solution
FROM busybox
ARG SETTINGS
RUN ./run/setup $SETTINGS
In this case when Dockerfile will be built with the flag --build-arg SETTINGS=--some-settings the flag --some-settings
will be passed to the RUN instruction.
Noncompliant code example
ARG SETTINGS="--default-settings"
FROM busybox
RUN ./run/setup $SETTINGS
In this case the $SETTINGS variable will be evaluated to empty string.
Compliant solution
ARG SETTINGS="--default-settings"
FROM busybox
ARG SETTINGS
RUN ./run/setup $SETTINGS
In this case the flag --default-settings will be passed to RUN instruction (unless another value is provided during build
time).
How does this work?
The FROM instruction starts a new build stage where variables defined by previous ARG instructions are out of this new
scope. To make it accessible for the build stage they need to be defined after the FROM instruction.
Resources
Documentation
