We run Dokku, and therefore dokku redis, in its own Docker container.
In order to make this work we map a path from the host into the
container as `/var/lib/dokku/services/redis`. Unfortunately the path
on the host is user-configurable, and generally _won't_ be the same
as the path in the container. This means that when we run `docker`
commands (e.g. to spin up Redis containers), the directory used for
bind mounts (the `-v` option) needs to be different.
This commit allows us to do this, but keeps the existing behaviour
(the redis root for Docker binds is the same as the redis root for
other uses) by default.
deb9317edf