**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
entrypoint=['sh', '-c'],
                    working_dir='/container/path',
                    tty=True,
                ),
            ]
        )
        self.client_mock.create_host_config.assert_has_calls(
            [
                call(
                    mounts=[
                        Mount(source='/host/path', target='/container/path', type='bind'),
                        Mount(source='/mkdtemp', target='/tmp/airflow', type='bind'),
                    ],
                    network_mode='bridge',
                    shm_size=1000,
                    cpu_shares=1024,
                    mem_limit=None,
                    auto_remove=False,
                    dns=None,
                    dns_search=None,
                    cap_add=None,
                    extra_hosts=None,
                    privileged=False,
                ),
                call(
                    mounts=[
                        Mount(source='/host/path', target='/container/path', type='bind'),
                    ],
                    network_mode='bridge',
                    shm_size=1000,