add hostname argument to DockerOperator

[skabbit]

DockerOperator could easily support hostname argument to pass in docker-py APIClient.create_container(). This could be useful in multiple cases, which we had run in.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
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[skabbit]

@uranusjr please, could you merge this PR?

[boring-cyborg]

Awesome work, congrats on your first merged pull request!