Add default VSS file to Dockerfile

[erikbosch]

With this PR the databroker container will by default load the VSS 3.0 file

erik@debian3:~/kuksa.val$ docker run --rm -it  -p 55555:55555/tcp databroker
2022-08-23T08:45:05.334442Z  INFO databroker: Init logging from RUST_LOG (environment variable not found)
2022-08-23T08:45:05.334473Z  INFO databroker: Starting Kuksa Data Broker 0.17.0
2022-08-23T08:45:05.334480Z  INFO databroker: Populating metadata from file 'vss_release_3.0.json'
2022-08-23T08:45:05.339142Z  INFO databroker: Listening on 0.0.0.0:55555
2022-08-23T08:45:05.339172Z  INFO databroker::broker: Starting housekeeping task
^C2022-08-23T08:45:12.903682Z  INFO databroker: received SIGINT
2022-08-23T08:45:12.903722Z  INFO databroker::grpc_service: Shutting down

[argerus]

So if arguments needs to be given then also the VSS 3.0 file needs to be included as argument

Instead of adding it as an argument in the docker file, you could set a default ENV for it.

That would make it:

1. possible to add additional arguments (without adding --metadata manually again).
2. easy to remove the default by manually setting an empty KUKSA_DATA_BROKER_METADATA_FILE.