Documentation on how to build the gitlab runner docker image itself?

Summary

Is there any documentation on how to build the gitlab runner docker image itself?

My company has a requirement that all third party open source software that we use must be downloaded in source form and built locally by company employees on company-owned equipment, using company-supported OSes (which is only RHEL derivatives).

Therefore, for me to use gitlab runner officially, I need to be able to build the gitlab runner docker image myself, using a RHEL-derivative OS.

Maybe I'm just looking in the wrong place, but I can't seem to find any documentation on how to actually build the gitlab runner docker image.

Does this documentation exist?