Is there an existing issue for this?

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Feature description

Docker has a feature called multi-stage builds where Docker images can be split up and referenced later by name.

Dependabot currently supports updates to images using the FROM directive, but naming the images may cause the update to not work properly.

Another related issue is that Dependabot does not currently update images that have two FROM directives in a Dockerfile, which is common with multi-stage builds, as are the COPY and ARG directives

We should confirm what level of support we have for Docker images.