Do not refer to an environment variable within the same ENV

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Metadata

ID: docker-best-practices/env-no-refer-envvar

Language: Docker

Severity: Info

Category: Best Practices

Description

This rule advises against referencing an environment variable within the same ENV instruction in a Dockerfile. For example, writing ENV FOO=bar \ BAZ=$FOO/bla is problematic because the Docker build process does not expand environment variables declared in the same ENV command.

To avoid this issue, declare each environment variable in a separate ENV instruction if you need to reference a previously set variable. For instance, use ENV FOO=bar followed by ENV BAZ=${FOO}/bla. This ensures that FOO is properly set and expanded when BAZ is defined, leading to more reliable and predictable builds.

Non-Compliant Code Examples

ENV FOO=bar \
    BAZ=$FOO/baz

ENV FOO=bar \
    BAZ=${FOO}/baz

Compliant Code Examples

ENV FOO=bar
ENV BAZ=${FOO}/baz