investigate pm2-runtime mode for Docker

[PM2](http://pm2.keymetrics.io/) is being used to keep process alive in the `lab` ([the main server](https://github.com/EpistasisLab/pennai/blob/master/dockers/lab/files/entrypoint.sh#L8) and [ai processes](https://github.com/EpistasisLab/pennai/blob/master/dockers/lab/files/entrypoint.sh#L39)) and `machine` ([the main server process](https://github.com/EpistasisLab/pennai/blob/ff808c7565c1376f4edba40b83fc77f056de9012/dockers/lab/files/entrypoint.sh#L39)) docker container instances.

PM2 has a [pm2-runtime](http://pm2.keymetrics.io/docs/usage/docker-pm2-nodejs/) mode optimized for working in production Docker environments. Investigate and possibly switch to pm2-runtime.

Also investigate not using PM2 for `machine` instances as they are managing only one process, and instead use the [docker restart policy](https://docs.docker.com/compose/compose-file/#restart_policy).