**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.tls_ssl_version = tls_ssl_version
        self.mount_tmp_dir = mount_tmp_dir
        self.tmp_dir = tmp_dir
        self.user = user
        self.mounts = mounts or []
        self.entrypoint = entrypoint
        self.working_dir = working_dir
        self.xcom_all = xcom_all
        self.docker_conn_id = docker_conn_id
        self.shm_size = shm_size
        self.tty = tty
        self.privileged = privileged
        self.cap_add = cap_add
        self.extra_hosts = extra_hosts
        if kwargs.get('xcom_push') is not None:
            raise AirflowException("'xcom_push' was deprecated, use 'BaseOperator.do_xcom_push' instead")
        self.cli = None
        self.container = None
    def get_hook(self) -> DockerHook:
        """
        Retrieves hook for the operator.
        :return: The Docker Hook
        """
        return DockerHook(
            docker_conn_id=self.docker_conn_id,
            base_url=self.docker_url,
            version=self.api_version,
            tls=self.__get_tls_config(),
        )
    def _run_image(self) -> Optional[str]: