title: airflow - how to get start date of current dag_run (not specific task)?
body: <p>Tasks 1, 2, 3, 4 in the same dag will insert to a db table.
  I then want task 7 to update the db table only for rows with timestamp &gt;= the time of the start of the dagrun (not the start time of task 7).</p>
  <p>Is there some jinja/kwarg/context macro i can use?
  I didn't see any example to get dagrun start_date (not exec date).</p>