Delete all data in Postgres database
You don't have to drop the database, it should be enough to drop all the objects in the database. This can be done using 

drop owned by adminuser


If you then create the SQL dump including the create table statements (so without the --data-only option) everything should be fine. 

You can also remove the --column-inserts then, which will make the import a lot faster.



However, if you do want to delete everything, you can do that with a little dynamic SQL:

do
$$
declare
  l_stmt text;
begin
  select 'truncate ' || string_agg(format('%I.%I', schemaname, tablename), ',')
    into l_stmt
  from pg_tables
  where schemaname in ('public');

  execute l_stmt;
end;
$$


This will truncate all tables in the schema public with a single statement which will also work even if there are many foreign key constraints connecting all tables. If your tables are spread over multiple schemas, you need to add them in the where condition.