separate run, data and static assets

[s-u]

With the new build system we have a nice separation of deployment and run-time. The docker compose setup shows that what we need are are three fundamental volumes: run for run-time sockets, pid files etc, data for persistent storage (rcs, sks, users data, ...) and static RCloud assets (in the images). Currently we only have ROOT but as #2749 points out we have to separate the static assets from run-time and data. So we need more locations that can be used in config files such, e.g. RCLOUD_ROOT, RCLOUD_RUN, RCLOUD_DATA - they could default to $ROOT, $ROOT/run and $ROOT/data respectively, but should be able to live outside of the $ROOT tree.