Add support for restart on watched files.

[memory]

If the env variable WATCH_FILES is set, use the entr(1) tool to wrap the entrypoint script to automatically restart the airflow process if a change is detected in any of the watched files.

The use case here is: a sidecar container syncs the dags directory from an external source, and that sync updates /usr/local/airflow/dags/requirements.txt: in this case we would want to restart the airflow process after re-processing the requirements.

[memory]

Slight nudge; is this functionality that you'd be interested in merging in?

[memory]

Poke. :)