Mount PostgreSQL Data from Persistent Volume

Mount PostgreSQL Data from Persistent Volume

Each time the postgresql container is torn down the database is lost.
By using a presistent volume managed outside of docker compose, the
postgresql data can persist across updates and service restarts.

The volume needs to exist before the postgres container can start: this
is done with the 'make data' command.

Note: postgres container creates a volume at /var/lib/postgresql/data.
Mounting at this location ensures the data is persisted into the
external volume.

Change-Id: I958ba57fc735c594378d575bc7108bee0d30c1e3
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>