Q: Can I change the value of /server/hostName property?
A: No, this is not allowed.
ectool setProperty /server/hostName ectool error \[PropertyNotSettable\]: '/server/hostName' is not a settable property Details: '/server/hostName' is not a settable property
'/server/hostName' is not a settable property
means that you cannot change the value of /server/hostName. It is one of the immutable properties.
When trying to detect the server host IP and name, CloudBees CD (CloudBees Flow) goes over the available interfaces and picks one.
That said, if you want to reference somecustomhostname:
either configure the IP address of the CloudBees CD (CloudBees Flow) server to resolve to somecustomhostname
or create your own property, set it to somecustomhostname, and use it instead of the hostName.