Adds container around merge request header

What does this MR do and why?

Keeps the users preferred container selection for the merge request header when viewing diffs. This is a follow-up to making the diffs view always use a fluid layout.

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How to set up and validate locally

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR. Closes #343042 (closed)