'mix ecto.create' errors with: role "postgres" does not exist

[romenigld]

Just create a user:
createuser --superuser postgres

check with the psql:

$ psql default
psql (14.1)
Type "help" for help.

default=# \du
                                   List of roles
 Role name |                         Attributes                         | Member of
-----------+------------------------------------------------------------+-----------
 postgres  | Superuser, Create role, Create DB                          | {}
 my_username | Superuser, Create role, Create DB, Replication, Bypass RLS | {}