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Comments for build stages or arguments should follow the description format.
PropertyValue
SeverityInfo
CategoryStyle
DefaultOff (experimental)
Auto-fixYes (--fix)

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Description

Comments for build stages or arguments should follow the format: # <arg/stage name> <description>. If a comment is not intended to be a description, add an empty line or comment between the instruction and the comment. The --call=outline and --call=targets flags for the docker build command print descriptions for build targets and arguments. The descriptions are generated from Dockerfile comments that immediately precede the FROM or ARG instruction and that begin with the name of the build stage or argument.

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Examples

Bad: 
# a non-descriptive comment
FROM scratch AS base

# another non-descriptive comment
ARG VERSION=1
Good (empty line separating): 
# a non-descriptive comment

FROM scratch AS base

# another non-descriptive comment

ARG VERSION=1
Good (proper description format): 
# base is a stage for compiling source
FROM scratch AS base
# VERSION This is the version number.
ARG VERSION=1

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Auto-fix

The fix inserts an empty line between the non-description comment and the instruction. 
# Before
# Some comment
FROM alpine AS builder

# After (with --fix)
# Some comment

FROM alpine AS builder

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Reference