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The AS and FROM keywords’ casing should match.
PropertyValue
SeverityWarning
CategoryStyle
DefaultEnabled
Auto-fixYes (--fix)

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Description

While Dockerfile keywords can be either uppercase or lowercase, mixing case styles is not recommended for readability. This rule reports violations where mixed case style occurs for a FROM instruction with an AS keyword declaring a stage name.

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Examples

Bad: 
FROM debian:latest as builder
FROM is uppercase but as is lowercase. Good: 
FROM debian:latest AS deb-builder

from debian:latest as deb-builder
Both keywords use the same casing.

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Auto-fix

The fix changes the AS keyword casing to match the FROM keyword. 
# Before
FROM alpine as builder

# After (with --fix)
FROM alpine AS builder

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Reference