chore: document how to migrate external database

apps/docs/pages/guides/self-hosting/docker.mdx

@@ -95,6 +95,10 @@ POSTGRES_PORT=5432
 
 Once you have done this, you can safely comment out the `db` section of the `docker-compose` file, and remove any instances where the services `depends_on` the `db` image.
 
+Supabase services require your external database to be initialized with a specific schema. Refer to our [postgres/migrations](https://github.com/supabase/postgres/tree/develop/migrations) repository for instructions on running these migrations.
+
+Note that you need superuser permission on the postgres role to perform the initial schema migration. Once completed, the postgres role will be demoted to non-superuser to prevent abuse.
+
 ### Setting database's `log_min_messages`
 
 By default, `docker compose` sets the database's `log_min_messages` configuration to `fatal` to prevent redundant logs generated by Realtime.