I tested the possibilities of the current mechanism of passing extra arguments to mockbuild first:

Mock can install extra packages via --additional-package. Example:

fedpkg --release=f37 mockbuild --no-clean-all -- --additional-package neovim --additional-package ripgrep

In your request, I can see --shell argument. Example above is not working with --shell because mock requires using --additional-package arguments in the "rebuild" mode (this is what fedpkg mockbuild does by default).

I found that possible solution might be running Mock sequentially 2 times. This is what https://pagure.io/rpkg/pull-request/704 does. Internally it runs:

fedpkg --release=f37 mockbuild --no-cleanup-after -- --additional-package neovim --additional-package ripgrep
fedpkg --release=f37 mockbuild --shell

Your scenario is executable (is it?) by current fedpkg's code after all and so far I am not convinced we need that code change above. Any comments?

Commit 89d3bb0d fixes this issue