Database is served via ./backups/data_backup.sql which applied by container init_db at the start of the app.
It's okay when you see init_db failed — in most recent case that means that DB is not empty — migrations are already applied. If it's not ok for you, you need to:

docker volume rm golang-gorm-postgres_postgres

The command clears docker volume, then you can apply db migrations.
And this is how you make a dump:

pg_dump -h localhost -p 6500 -U postgres -f data_backup.sql golang-gorm

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