**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
if add_tmp_variable:
            self.environment['AIRFLOW_TMP_DIR'] = self.tmp_dir
        else:
            self.environment.pop('AIRFLOW_TMP_DIR', None)
        self.container = self.cli.create_container(
            command=self.format_command(self.command),
            name=self.container_name,
            environment={**self.environment, **self._private_environment},
            host_config=self.cli.create_host_config(
                auto_remove=False,
                mounts=target_mounts,
                network_mode=self.network_mode,
                shm_size=self.shm_size,
                dns=self.dns,
                dns_search=self.dns_search,
                cpu_shares=int(round(self.cpus * 1024)),
                mem_limit=self.mem_limit,
                cap_add=self.cap_add,
                extra_hosts=self.extra_hosts,
                privileged=self.privileged,
            ),