IPv6 is definitely causing problems, but for better or worse the way forward is IPv6. My personal opinion is that if all software on the internet needs to be dragged kicking and screaming into the future, so be it 
How to connect to a 3rd party image that listens for IPv4 addresses using Fly's private networking? - #16 by uncvrd
And yeah, setting the IPv6 address to the host should work. The problem is not with using IPv6 itself, it’s the resolution of these addresses from a abc.internal domain name.