The entrypoint-artifactory.sh file does a waitForDB check to access the database by parsing DB_URL.
For a URL that uses a TNS entry this doesn't work, presumably because the hostname is in the tnsnames.ora and not the URL. To get around this one has to modify the entry point and remove the check.
Artifactory standalone in fact works fine with this DB configuration.
A possible solution would be to include a property that tells the startup to ignore this check (DB_CHECK_IGNORE) or find some other way to verify the DB is up.