Well today is close to two months that my Pi cluster has been up and running. For the most part it has worked without any issues. Once Portainer was up and running I went about installing the different functions I wanted to have on each node. The first thing I thought of was a new Nagios container. I have used Nagios on my own network and at a data center where I worked. This was by far the most difficult of the installs. To get the current version you have to build it from source and then go about configuring.  Here is an old but good guide to building a Nagios server.