Clarify docs on Dependency Proxy authentication

What does this MR do?

Our documentation mentioned that Automatic runner authentication added in GitLab 13.9.

I believe this means that the runner can automatically pull the image for the job without us configuring the GitLab Runner to authenticate to GitLab.

However, this confused one of our customers as they thought that they can use the Dependency Proxy via the Dockerfile when building a Docker image without adding a docker login in their CI.

I clarified the wording and also added an example on how to use Dependency Proxy when building an image to prevent any confusion.

Related issues

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