**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

airflow/providers/docker/operators/docker.py
self,
        *,
        image: str,
        api_version: Optional[str] = None,
        command: Optional[Union[str, List[str]]] = None,
        container_name: Optional[str] = None,
        cpus: float = 1.0,
        docker_url: str = 'unix://var/run/docker.sock',
        environment: Optional[Dict] = None,
        private_environment: Optional[Dict] = None,
        force_pull: bool = False,
        mem_limit: Optional[Union[float, str]] = None,
        host_tmp_dir: Optional[str] = None,
        network_mode: Optional[str] = None,
        tls_ca_cert: Optional[str] = None,
        tls_client_cert: Optional[str] = None,
        tls_client_key: Optional[str] = None,
        tls_hostname: Optional[Union[str, bool]] = None,
        tls_ssl_version: Optional[str] = None,
        mount_tmp_dir: bool = True,
        tmp_dir: str = '/tmp/airflow',
        user: Optional[Union[str, int]] = None,
        mounts: Optional[List[Mount]] = None,
        entrypoint: Optional[Union[str, List[str]]] = None,
        working_dir: Optional[str] = None,
        xcom_all: bool = False,
        docker_conn_id: Optional[str] = None,