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ENV key=value should be used instead of the legacy ENV key value format.
PropertyValue
SeverityWarning
CategoryStyle
DefaultEnabled
Auto-fixYes (--fix)

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Description

The correct format for declaring environment variables and build arguments in a Dockerfile is ENV key=value and ARG key=value, where the variable name and value are separated by an equals sign. Historically, Dockerfiles have also supported a space separator. This legacy format is deprecated.

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Examples

Bad: 
ARG foo bar
Good: 
ARG foo=bar
Bad: 
ENV DEPS \
    curl \
    git \
    make
Good: 
ENV DEPS="\
    curl \
    git \
    make"

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Auto-fix

The fix replaces the whitespace-separated format with equals format. Quotes are added if the value contains spaces. 
# Before
ENV key value
ENV multi word value

# After (with --fix)
ENV key=value
ENV multi="word value"

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Reference