**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
dns: Optional[List[str]] = None,
        dns_search: Optional[List[str]] = None,
        auto_remove: bool = False,
        shm_size: Optional[int] = None,
        tty: bool = False,
        privileged: bool = False,
        cap_add: Optional[Iterable[str]] = None,
        extra_hosts: Optional[Dict[str, str]] = None,
        **kwargs,
    ) -> None:
        super().__init__(**kwargs)
        self.api_version = api_version
        self.auto_remove = auto_remove
        self.command = command
        self.container_name = container_name
        self.cpus = cpus
        self.dns = dns
        self.dns_search = dns_search
        self.docker_url = docker_url
        self.environment = environment or {}
        self._private_environment = private_environment or {}
        self.force_pull = force_pull
        self.image = image
        self.mem_limit = mem_limit
        self.host_tmp_dir = host_tmp_dir
        self.network_mode = network_mode
        self.tls_ca_cert = tls_ca_cert
        self.tls_client_cert = tls_client_cert
        self.tls_client_key = tls_client_key
        self.tls_hostname = tls_hostname