So I'm a linux noob which is why I can't get this to work.
I already know that I need to set IPV6_ADDRESS= in /etc/pihole/setupVars.conf but how do I actually do that. I opened the cli of pihole in Docker Desktop but things i googled like "sudo vim /etc/pihole/setupVars.conf" get me "command not found".
How and where do I do this?
Thanks a lot.
Even in a Pi-hole bare metal configuration, setupVars.conf is not intended to be edited manually. Run pihole -r with Reconfigure instead.
Yet in a dockered Pi-hole, you should never use pihole -r.
If you want to customise your container, do so via Pi-hole's corresponding Docker environment variables.
With regards to IPv6, you should be aware that by default, Docker's networking is IPv4 only.
Check Docker's documentation on how to change that.
If you would run Docker for Windows, then there's currently no IPv6 support at all.
Well thats a lot of if's and don'ts 
Maybe you have an idea how to solve my main problem:
I have iOS devices that use ipv6 instead of my manually configured ipv4 dns on them (because who knows why) to talk to my router instead of my pihole windows server and then go out and show ads.
apparently its not possible to disable dhcpv6 fully on my fritzbox (i did and devices receive ipv6 address anyway) which is why i wanted to configure a static ipv6 dns in dhcpv6 so that the iOS devices use the correct dns server.
Then the fritzbox will always advertise itself as a DNS server. Adding a second DHCPv6 server will not remove the one on fritzbox and will cause you more headaches. You can have only one authoritative DHCPv6 server on any network segment.
I have iOS devices that use ipv6 instead of my manually configured ipv4 dns on them
Find out how to disable IPv6 on that device.
Disabling IPv6 is possible with a FritzBox.
See my hint in your related topic, where you claim you haven't tried disabling IPv6 yet.