**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
#
#
#
import logging
import unittest
from unittest import mock
from unittest.mock import call
import pytest
from docker.errors import APIError
from airflow.exceptions import AirflowException
try:
    from docker import APIClient
    from docker.types import Mount
    from airflow.providers.docker.hooks.docker import DockerHook
    from airflow.providers.docker.operators.docker import DockerOperator
except ImportError:
    pass
TEMPDIR_MOCK_RETURN_VALUE = '/mkdtemp'
class TestDockerOperator(unittest.TestCase):