Is there any documentation on creating custom runners?

I’m looking into creating a custom runner (specifically one that runs on Linode VPSs), but I’m not really sure where to start.

I saw this GitHub Organization and this template repository, but neither seemed to fit my use case;

My goal is to create one that works via a shell script, but again, I’m not sure what I’d need to get everything going.

I saw in the temp folder some stuff that involved daemons, which is leading me to think that the runner would require a web server or something similar attached to it?

Anywhere I can look at to attempt to start somewhere? Would it be best to just look at the examples I provided and try to hack something from that?