**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
operator = DockerOperator(
            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')],
            entrypoint='["sh", "-c"]',
            working_dir='/container/path',
            shm_size=1000,
            host_tmp_dir='/host/airflow',
            container_name='test_container',
            tty=True,
        )
        operator.execute(None)
        self.client_class_mock.assert_called_once_with(
            base_url='unix://var/run/docker.sock', tls=None, version='1.19'
        )
        self.client_mock.create_container.assert_called_once_with(
            command='env',
            name='test_container',
            environment={'AIRFLOW_TMP_DIR': '/tmp/airflow', 'UNIT': 'TEST', 'PRIVATE': 'MESSAGE'},
            host_config=self.client_mock.create_host_config.return_value,
            image='ubuntu:latest',
            user=None,
            entrypoint=['sh', '-c'],