Amplifying on this, some basic postgres commands once you have the postgres command line in the container:
list all databases
\l
select a database
\c dbname
list tables in the current database
\dt
list columns in the users table
\d+ users
number of rows in the users table
select count(*) from users;
display value of configuration variables
show shared_buffers;
exit from the postgres console
\q
If you know SQL I am sure you can take it from here 
note that all SQL must terminate with semicolon ;