A DNS server can provide A (IPv4) and AAAA (IPv6) records regardless.
As far as DNS lookups are concerned, you are not losing anything by running Pi-hole only on IPv4.
However, if your router advertises itself as DNS server via IPv6, clients will by-pass Pi-hole via IPv6.
Yet that wouldn't stop if your Pi-hole had an IPv6 address: Your Pi-hole would advertise itself, and a client would then pick your router's or Pi-hole's IPv6 at its own discretion.
The only way to avoid an IPv6 by-pass is configuring your router to not distribute its own IPv6 address for DNS. If your router doesn't support that, your only other options to avoid by-passes would be to turn off IPv6, or to redirect DNS traffic to your Pi-hole via respective rules in your router's firewall or a dedicated firewall (if you happen to run one).
If you still want IPv6 for Docker, you probably should start by checking whether IPv6 is enabled for your Docker installation (which isn't available for all platforms), see Docker's documentation at Enable IPv6 support | Docker Documentation.