**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
hosts_obj = mock.Mock()
        operator = DockerOperator(task_id='test', image='test', extra_hosts=hosts_obj)
        operator.execute(None)
        self.client_mock.create_container.assert_called_once()
        assert 'host_config' in self.client_mock.create_container.call_args[1]
        assert 'extra_hosts' in self.client_mock.create_host_config.call_args[1]
        assert hosts_obj is self.client_mock.create_host_config.call_args[1]['extra_hosts']
    def test_privileged(self):
        privileged = mock.Mock()
        operator = DockerOperator(task_id='test', image='test', privileged=privileged)
        operator.execute(None)
        self.client_mock.create_container.assert_called_once()
        assert 'host_config' in self.client_mock.create_container.call_args[1]
        assert 'privileged' in self.client_mock.create_host_config.call_args[1]
        assert privileged is self.client_mock.create_host_config.call_args[1]['privileged']