How to run a Git command with a config specified in a separate file? [duplicate]

I can run a Git command with -c <name>=<value> switch to specify a single configuration parameter, e.g.

git -c user.name="John Doe" commit 

Now I have a config file with multiple options, which I want a particular command to use in the same way. How can I achieve this?

Edit: I don't want to specify a custom global .gitconfig. I want to use a file to override/add some config options for a particular Git command.