Ok... I've solves my specific Go issue, but not my generic root cert issue/questions.
Based on this link:
https://stackoverflow.com/questions/40051213/where-is-golang-picking-up-root-cas-from/40051432#40051432it shows that Go is looking in certain locations for the cert bundle, so doing this addresses the specific problem I was having:
wget
https://curl.se/ca/cacert.pemsudo mkdir -p /etc/ssl/certs
sudo mv ./cacert.pem /etc/ssl/certs/ca-certificates.crt
(unsure why I can't make the above into a code block!?! The forum software gives me an internal server error when I do)
All I've done there is grabbed the latest official (or at least extracted from official) Mozilla CA cert bundle and put it in the debian/ubuntu location where Go will look for it.
I'm not sure about how to (or whether it is wise to) unpack that bundle into all the individual certs that exist in the ca-certificates.tcz package.
Muddling my way through this it seems a bit shocking to me how non-standardized (and confusing) this is. It's weird that there are both non-standard locations for the bundle file, as well as seemingly mixed usage of whether applications need/use the bundle or individual certs.