DL4000 - MAINTAINER is deprecated

Description

The MAINTAINER instruction is obsolete. Declare the image author with LABEL maintainer="name" instead.

Goals

• Remove usage of the deprecated MAINTAINER instruction.
• Promote explicit authorship via a LABEL directive.

Specification

1. Walk each top-level instruction in the Dockerfile AST.
2. If an instruction's keyword equals MAINTAINER (case-insensitive), emit DL4000.
3. Report the line of the MAINTAINER instruction with the message MAINTAINER is deprecated. Use LABEL maintainer="name" instead.

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