LegacyKeyValueFormat - Docker

Output

text

"ENV key=value" should be used instead of legacy "ENV key value" format

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 (key) and value (value) are separated by an equals sign (=). Historically, Dockerfiles have also supported a space separator between the key and the value (for example, ARG key value). This legacy format is deprecated, and you should only use the format with the equals sign.

Examples

❌ Bad: using a space separator for variable key and value.

dockerfile

FROM alpine
ARG foo bar

✅ Good: use an equals sign to separate key and value.

dockerfile

FROM alpine
ARG foo=bar

❌ Bad: multi-line variable declaration with a space separator.

dockerfile

ENV DEPS \
    curl \
    git \
    make

✅ Good: use an equals sign and wrap the value in quotes.

dockerfile

ENV DEPS="\
    curl \
    git \
    make"