Give access to /srv/pad to etherpad user

Summary:
The Etherpad container now runs the Node process as an unprivileged user.

Ensure the files it needs to access belongs to same 9001 user.

This is a follow-up for rDETHER3232c24aca4a.

Ref T1629.

Test Plan: Tested on docker-001

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1629

Differential Revision: https://devcentral.nasqueron.org/D2325