[LabKey Cloud] Dockerfile: update to use canonical log4j2.xml if custom not specified

[labkey-willm]

Previously, you could set the LOG4J_CONFIG_FILE env var to the canonical log4j2.xml file's path to make it use that instead of the local customized version: https://github.com/LabKey/Dockerfile/blob/develop/entrypoint.sh#L17

... but that file is no longer present. If the -Dlog4j.configurationFile= arg is not present, the container should use the cononical one instead. This needs to be verified, and, if true, enabled as an option.

So, as we've done with other options like DD_JAVA_AGENT, make a new var that either contains the full arg and path (when defined), or else is blank