proposal: Support ARG --description flag for inline build argument documentation

[janishorsts]

Use case

Modern Dockerfile and BuildKit specifications (Dockerfile v1.8+) support the --description flag on ARG instructions:

ARG --description="Target binary release version" VERSION=1.0.0

Earthfile currently supports --required and --global options on ARG (earthfile.abnf:L308), but lacks explicit --description="..." support.

Adding ARG --description="..." to Earthfile will allow recipe authors to document build arguments directly in code, improving developer experience and integrating seamlessly with EarthBuild's CLI documentation tool (earth doc), which inspects target argument descriptions.

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Expected Behavior

Usage in Earthfiles

build:
    ARG --description="Environment stage (dev, staging, prod)" ENV=prod
    ARG --required --description="Database connection URL" DB_URL

    FROM alpine:3.18
    RUN echo "Building for ${ENV}"

CLI Documentation Integration (earth doc)

earth doc +build automatically renders argument descriptions provided via --description="...".

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Grammar Specification (earthfile.abnf)

Update arg-option in internal/earthfile/earthfile.abnf:

arg-option          = "--required" / "--global" / ( "--description" "=" ( WORD / QUOTED-STRING ) )