I don’t know well enough to tell you.
You’d need to crank up some container, enter it, and then try to git clone or maybe even just curl https://github.com/.
I suspect for your IPv6-only setup it’s going to be difficult and unsupported. I with google git+clone+inside+docker+ipv6 I found this: tasks have no access to ipv6 services · concourse · Discussion #5919 · GitHub