I just tested locally and it does seem that restarting the Nginx container wipes the configuration. I wasn’t sure if this would happen and assumed it would not.
Since this is the default behavior, I would recommend putting in a feature request here for the ability to upload custom nginx rules from DevKinsta that will stick on restarts so our developers can review that for future releases.
In the meantime, I did find a workaround but it means you have to insert the data any time you restart DevKinsta or the Nginx docker container.
My fix was to edit the file, then, instead of restarting the container, run this command and exit the terminal window.
nginx -s reload