1) Create a linux user and set password
  sudo adduser read_access

  .. then enter your password for this user

2) Limit permission for the user (option)
  .. normally, this user only has root permission on it's directory. (/home/read_access)

3) Set read-only role for this user to use Postgresql

  psql -U postgres -d <your database>

  CREATE USER read_access WITH PASSWORD '<your password>';

  GRANT CONNECT ON DATABASE <your database> TO read_access;

  GRANT SELECT ON ALL TABLES IN SCHEMA public TO read_access;

  GRANT SELECT ON ALL SEQUENCES IN SCHEMA public TO read_access;

  ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT SELECT ON TABLES TO read_access;

  ALTER USER read_access SET default_transaction_read_only = on;

  <br/>
4) Dump your database
   PGPASSWORD="<your password>" pg_dump -Fc --no-acl --no-owner -h localhost -U read_access -d <your database> -t <your table> > mydb.dump

5) If you try to "change you will get this error"
ERROR:  cannot execute INSERT in a read-only transaction