Label and persist the "artifactory_extra_conf" Docker volume even if it is empty

Currently, the Artifactory Docker image contains a separate volume for the artifactory_extra_conf folder. If a user doesn't explicitly set the volume, the Docker CLI still reserves the space as a Volume:

$ docker volume ls

local 38f7c0ba8d2f8dca7c0013306266566f20ed7899695290e5c68a05345f18102f

local 205afe465683cdc02b469129a291c261ab96a8898e15f98277ffde710612aa8e

local 9475de101d2ae6252fb11e5b3bab323193b578b4f8a32e805c01220afa30ae5f

Another unlabeled volume is created whenever the container is created or restarted. This volume should be labeled so it is persisted.