Where's Arnold in Docker Maya 2018?

I’m using Docker Maya 2018 on Centos. When I’m in docker’s mayapy interpreter and I load Arnold cmds.loadPlugin("mtoa"), I get this error:
Error: file: /root/workdir/tests/ma/result.ma line 7: RuntimeError: file <maya console> line 1: Plug-in, "mtoa", was not found on MAYA_PLUG_IN_PATH.
Where is mtao.so in this docker image?
My hunch is that I would set MAYA_PLUG_IN_PATH as a variable in the dockerfile. Your thoughts?