Mount Airflow volumes in maintenance container too

Summary:
New DAGs can be debug from a maintenance Airflow container.
To do so, the same volumes than for the services need to be there.

Ref T1812.

Test Plan: ls dags

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1812

Differential Revision: https://devcentral.nasqueron.org/D3284