Beginning to wonder if Packer is the right tool for the job?

I’ve been tasked with using Packer to create up-to-date VMware templates in an air-gapped network environment, but as far as I can see, Packer wants to download the Linux ISO from the Internet each time I run the script? OK, I gather it will “cache” the ISO once it has downloaded it once, and I can put the ISO on an internal http server, but now it has done that it seems to want to create an ISO, because I have another error:

could not find a supported CD ISO creation command (the supported commands are: xorriso, mkisofs, hdiutil, oscdimg)

I already downloaded an ISO and have it on a DataStore, so why does it want to create another?!

All I want to do (sounds simple), is build a VM, update it (from an internal mirror), shut it down and create a template. Is that too much to ask?