[AIRFLOW-2891] Make DockerOperator container_name be templateable

[ashb]

Make sure you have checked all steps below.

Jira

• https://issues.apache.org/jira/browse/AIRFLOW-2891

Description

• Further to [AIRFLOW-2891] Option to set container_name for DockerOperator #5689, but lets also allow the docker container name to be templated so that it can be changed per task intances.

Tests

• templates in generally are well tested.

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

Code Quality

• Passes flake8

[ashb]

cc @rocketbitz

[BasPH]

LGTM