def umount(self, all=False, force=True):
        """
        unmount container filesystem
        :param all: bool, option to unmount all mounted containers
        :param force: bool, force the unmounting of specified containers' root file system
        :return: str, the output from cmd
        """
        # FIXME: handle error if unmount didn't work
        options = []
        if force:
            options.append('--force')
        if all:
            options.append('--all')
        cmd = ["podman", "umount"] + options + [self.get_id() if not all else ""]
        return run_cmd(cmd, return_output=True)