DL3007 : Avoid latest tag

Description

FROM instructions should specify a version or digest instead of relying on the implicit or latest tag. Pinning image versions ensures reproducible builds.

Goals

• Promote reproducible builds by avoiding floating latest tags.
• Encourage explicit version pinning for base images.

Specification

1. For each stage's base image in FROM, ignore images with a digest (@).
2. Split the image reference on :.
3. If no tag is present or the tag equals latest, emit DL3007 at the FROM line.

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