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


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.


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

FROM alpine
ARG foo bar

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

FROM alpine
ARG foo=bar