adjust docker folder naming convention to have similar layout as NiFi

Currently NiFi's docker image (https://github.com/apache/nifi/blob/41663929a4727592972e6be04b3c516a752e760e/nifi-docker/dockerhub/Dockerfile#L32) installs NiFi ( and sets that to $NIFI_HOME) into a version agnostic folder.

NiFi registry's docker image 

https://github.com/apache/nifi-registry/blob/f3b82a7b8dab3737d9b9ca1dc8028d4e9d7108fa/nifi-registry-core/nifi-registry-docker/dockerhub/Dockerfile#L29

does it differently: it always creates a version specific folder (and sets that to $NIFI_REGISTRY_HOME) for any new NiFi Registry releases.

For NiFi Registry, I'd like to propose, to follow the same convention, in order to make it easier for downstream use-cases to find NiFi Registry's home folder / directory layout