By default configuration of PostgreSQL user called postgres and it comes with full super-admin access to entire PostgreSQL instance running on your operating system or in the Container. The below commands are helpful to create a database, username, password and grant permission for the user to access the specific database. 

As we discussed above the postgres user has super-admin permission so switch user to postgres,

$ sudo su - postgres

Enter into the postgres database 
 
$ psql

Then, execute the following commands, create a new database,
 
psql=#  CREATE DATABASE <DATABASE>;

Create a user account with encrypted password,
 
psql=#  CREATE USER <USER> WITH ENCRYPTED PASSWORD '<PASSWORD>';

Set grant privilege for the user to access the database,
 
psql=#  GRANT CONNECT ON DATABASE <DATABASE> TO <USER>;

If you would like to change the password run the command below,
 
psql=#  ALTER USER <username> WITH ENCRYPTED PASSWORD  '<PASSWORD>';