I used these instructions by Matt Hisk to deploy this ghost blog to my dokku instance, with a few modified directions related to setting the database configuration.

First, get the DATABASE_URL after running this command:

dokku mysql:link my-ghost-db my-ghost-app

That link will look something like this:

mysql://<USER>:<PASSWORD>@<HOST>:3306/<DATABASE>

You will need to set the config variables for the ghost app as such:

dokku config:set my-ghost-app \
  database__client=mysql \
  database__connection__host=<HOST> \
  database__connection__user=<USER> \
  database__connection__password=<PASSWORD> \
  database__connection__database=<DATABASE>