Add DOKKU_ROOT_ON_HOST variable

This is to be used in place of `DOKKU_ROOT` when we need to refer to a
path on the system where the Docker daemon is being run; if `dokku` is
running in a container this may be different from the regular
`DOKKU_ROOT`.

dokku

@@ -27,6 +27,7 @@ if [[ -d $DOKKU_ROOT/.dokkurc ]]; then
   done
 fi
 [[ $DOKKU_TRACE ]] && set -x
+export DOKKU_ROOT_ON_HOST=${DOKKU_ROOT_ON_HOST:=$DOKKU_ROOT}
 
 export DOKKU_DISTRO
 # shellcheck disable=SC1091