Updates the `build-images` workflow to robustly detect and validate changes to image directories.
Changes:
- Implements strict validation to ensure detected image directories exist and contain a `Dockerfile` before adding them to the build matrix.
- Normalizes logic for both manual inputs and automatic git-diff detection using a temporary file (`valid_images.txt`) to collect targets.
- Adds specific checks to warn if a manually specified image is missing or invalid.
- Prevents the build job from failing or running unnecessarily if an empty image name is passed to the matrix by adding an early exit condition.
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