When no airflow files change, selective tests only run basic
tests, but this is wrong, because many of .py files are
outside of the airflow folder.
In this case we should enable image building because only then
full set of static checks is executed.
This bug caused for example #13403 to succeed even if it failed
static checks after merge.
(cherry picked from commit 1fe83a435d)
We had a lot of problems recently about the queues in Github
Actions. This documentations explains the motivation and approach
we have taken for optimizing our PR workflow.
Jarek Potiuk
John Bampton