[-] remove misleading VOLUME /var/lib/postgresql declaration (#780)

PostgreSQL base image already defines `VOLUME /var/lib/postgresql/data`.
Adding another volume for `/var/lib/postgresql` causes confusion and
may lead to unexpected behavior when mounting persistent data volumes.

Author: pashagolub

docker/demo/Dockerfile

@@ -86,7 +86,6 @@ EXPOSE 9187
 ### using traditional means is still recommended before updating - see "Updating to a newer Docker version" from README
 
 VOLUME /pgwatch/persistent-config
-VOLUME /var/lib/postgresql
 VOLUME /var/lib/grafana