Execute a command inside a running container as the postgres user,
        asserting success.

def exec_pg_success(self, cmd):
        """
        Execute a command inside a running container as the postgres user,
        asserting success.
        """
        result = self.inner().exec_run(cmd, user='postgres')
        assert result.exit_code == 0, result.output.decode('utf-8')
        return result