**Apache Airflow version**: 2.0.0

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): NA

**Environment**: 
* local Ubuntu 18.04 LTS / 
* docker-compose version 1.25.3, build d4d1b42b
* docker  20.10.1, build 831ebea

**What happened**:

when enabling xcom push for a docker operator the following error is thrown after the task finishes succesfully:
`TypeError: Object of type bytes is not JSON serializable`


**What you expected to happen**:

* error is not thrown
* if xcom_all is True: xcom contains all log lines
* if xcom_all is False: xcom contains last log line


**How to reproduce it**:

see docker compose and readme here:
https://github.com/AlessioM/airflow-xcom-issue

tests/providers/docker/operators/test_docker.py
self.client_mock.pull.return_value = [b'{"status":"pull log"}']
        kwargs = {
            'api_version': '1.19',
            'command': 'env',
            'environment': {'UNIT': 'TEST'},
            'private_environment': {'PRIVATE': 'MESSAGE'},
            'image': 'ubuntu:latest',
            'network_mode': 'bridge',
            'owner': 'unittest',
            'task_id': 'unittest',
            'mounts': [Mount(source='/host/path', target='/container/path', type='bind')],
            'working_dir': '/container/path',
            'shm_size': 1000,
            'host_tmp_dir': '/host/airflow',
            'container_name': 'test_container',
            'tty': True,
        }
        xcom_push_operator = DockerOperator(**kwargs, do_xcom_push=True)
        no_xcom_push_operator = DockerOperator(**kwargs, do_xcom_push=False)
        xcom_push_result = xcom_push_operator.execute(None)
        no_xcom_push_result = no_xcom_push_operator.execute(None)
        assert xcom_push_result == b'container log'
        assert no_xcom_push_result is None
    def test_extra_hosts(self):