Move Docker root directory for Dwellers

Summary:
Docker engines has a dedicated disk for Docker data and data volumes.

With overlay2, it means only a partition mounted as /srv.
We can store Docker images and volumes there using /srv/docker.

Ref T1769

Test Plan: Apply to dwellers

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1769

Differential Revision: https://devcentral.nasqueron.org/D2796