buildkit/UndefinedArgInFrom

FROM argument is not declared.

Property	Value
Severity	Warning
Category	Correctness
Default	Enabled

Description

This rule warns for cases where you are consuming an undefined build argument in FROM instructions. Interpolating build arguments in FROM instructions can be a good way to add flexibility to your build. However, if the argument is never declared with ARG, the variable silently resolves to an empty string, which is almost certainly not the intended behavior. This check also tries to detect and warn when a FROM instruction references misspelled built-in build arguments, like BUILDPLATFORM.

Examples

Bad:

FROM node:22${VARIANT} AS jsbuilder

Good:

ARG VARIANT="-alpine3.20"
FROM node:22${VARIANT} AS jsbuilder

You can also pass the argument at build time:

docker buildx build --build-arg ALPINE_VERSION=edge .

Reference