First instruction should be ARG or FROM

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Metadata

ID: docker-best-practices/first-instruction

Language: Docker

Severity: Warning

Category: Best Practices

Description

This rule enforces that the very first instruction in a Dockerfile must be either ARG or FROM. These instructions are fundamental for defining the build-time variables and the base image, respectively, which are crucial for the Docker build process to function correctly. Starting with ARG or FROM ensures that the build context is properly initialized before any other commands are executed. This helps avoid unexpected behaviors or errors during the build, as Docker needs a base image or build argument context to interpret subsequent instructions.

Non-Compliant Code Examples

COPY foo bar

ADD foo bar

Compliant Code Examples

ARG foo
RUN something

# Something
FROM debian:jessie
RUN foobar

FROM debian:jessie
RUN foobar