Introduce DOCKER_VOLUMES_PATH_PREFIX

The idea behind this is for each Docker volume that contains some
part of the state of RDMS to be bind-mounted from a hardcoded
subdirectory of a well-defined, configurable path outside of
/var/lib/docker.

This should make it easier and less error-prone to create backups of and
RDMS instance as a whole, especially when compared to Docker's official
recommendation of creating a tarball of each volume individually using
"docker run":
https://docs.docker.com/storage/volumes/#backup-restore-or-migrate-data-volumes