**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
tls_mock = mock.Mock()
        tls_class_mock.return_value = tls_mock
        operator = DockerOperator(
            docker_url='tcp://127.0.0.1:2376',
            image='ubuntu',
            owner='unittest',
            task_id='unittest',
            tls_client_cert='cert.pem',
            tls_ca_cert='ca.pem',
            tls_client_key='key.pem',
        )
        operator.execute(None)
        tls_class_mock.assert_called_once_with(
            assert_hostname=None,
            ca_cert='ca.pem',
            client_cert=('cert.pem', 'key.pem'),
            ssl_version=None,
            verify=True,
        )
        self.client_class_mock.assert_called_once_with(
            base_url='https://127.0.0.1:2376', tls=tls_mock, version=None
        )
    def test_execute_unicode_logs(self):