Explain ARG IMAGE= updates in dockerfile manager docs

[HonkingGoose]

Describe the proposed change(s).

Todos:

• Read the code snippets from the maintainer: Documentation for ARG IMAGE= in dockerfile manager #28916 (comment)
• Improve the Renovate docs, Dockerfile manager so it explains how you can get Renovate to update the ARG IMAGE= parts (use the pencil icon on the published docs page to get to the correct file)

I don't understand the code enough to write the docs improvements, so I'll let someone else pick up this issue. 😉