How do I enable DKIM for non-hosted Discourse emails?

I’d like to set up DKIM to help ensure that email notifications from my self hosted Discourse instance actually get reliably delivered.

DomainKeys Identified Mail (DKIM) lets an organization take responsibility for a message that is in transit. The organization is a handler of the message, either as its originator or as an intermediary. Their reputation is the basis for evaluating whether to trust the message for further handling, such as delivery. Technically DKIM provides a method for validating a domain name identity that is associated with a message through cryptographic authentication.

How do I do this?

Source: https://meta.discourse.org/t/how-do-i-enable-dkim-for-non-hosted-discourse-emails/5487