It is currently not possible to somehow initiate a build in GitLab CI based on which files have been edited.
I had thought of this feature before, and I think it can be very useful for several different use cases.
The closest problem I could find for this is https://gitlab.com/gitlab-org/gitlab-ce/issues/23010
In any case, to answer your question:
You cannot do it right now - even manually - because there is no way to determine if the merge request has changed the file or not, since CI has no concept of a merge request.
Pipelines βunderstandβ that merger requests should be added soon.
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