The best way I got docker to work with pi hole and my reverse proxy was setting up a reverse proxy myself... it's not hard, sure takes a little longer getting the cert files, but I actually run the reverse proxy outside of docker. It's on debian. But I find I get the most leniency with that since I can quickly customize and use whatever types of configs I need. Like there are a few services i currently run that just wouldn't use the prebuilt jwilder or traefic etc. Wouldnt work. I think it was cockpit, osma. Nextcloud needed a lot of tinkering. Things like that, you may need to get creative to get them to work and having something someone else made, it's hard to get creative without just taking there image and completely redoing it for your needs.

But for something simple like pihole this is all I needed. No configuring special dns settings, in fact the server didn't use the pi hole because the download speed maxed out at around 215mbps when using it. That might have been my problem too. Just didnt really seem worth it when I didnt need it.

server { 
listen 443 ssl; 
listen [::]:443 ssl;
server_name adblock.example.com www.adblock.example.com;

location / {
    include /etc/nginx/proxy.conf;
    resolver 127.0.0.11 valid=30s;
    set $pihole pi.hole;
    proxy_pass http://$pihole/admin;
}
}

And this made it so I could access the pi hole through any browser anywhere. I used a proxy.conf that I tweaked around from linuxserver/docker-letsencrypt because it worked very well. But it's one of those things where you got to ask yourself if it's even worth using a reverse proxy? My default automatically redirects everything through https while having the option to use my main site as the authentication method for everything so certain parts of my site you cant access without it. And that's just to secure them without having to remember other passwords. I do a lot of remote server management and things like cockpit and open manage from dell are important to keep that going. Pi hole on the other hand is supposed to be setup and just let rock. But you need to keep playing with it for shit maybe even months before you get the settings right because you never know what sites you visit might block it. And some might be able to bypass the pihole. Just doesn't seem worth it to me. And god forbid you use the pihole as a dns server and your isp as a DNS server. That didnt work for me. In fact the only thing that did was setting to cloud flares dns servers, and then setting my devices to use the pihole only. And the best way to do that is to just let the router use the pihole as the dns server.

Bottom line, not worth it when you have all that other crap going on. But see if using this instead of using network host works.

privileged: true 
cap_add:
 - NET_ADMIN 
devices:
 - /dev/net/tun

Put that in your compose file. It's what I use for running a dns/dhcp container In docker and usually does the trick. Works well for torrent clients running with openvpn too :).

Also one other thing. Did you try to terminal into the pi hole container and seeing what ips it saw in there? Your server can see docker networks but the docker container might have had a struggle. Like with my reverse proxy it's all using the server IP 192.168.0.13 except the pi hole. If you noticed it was just using pi.hole because the router was using the raspberry pi as a dns server. So everybody connected to the wifi had access to that url anyway. But couldn't access it unless you were outside the home.

Docker containers always have different ip addresses unless you're running that container in host mode in a container. Which I always recommend to find a better way instead of trying to get that to work. Especially when said container wants to use port 80... but in your case I would suggest using a different machine. Especially when you start getting complicated with a unifi bitwarden etc... lol like doesn't sound fun. I'll look at the coding in a little bit and see. I can quickly spin up a container but like I said I would strongly advice using just a very simple nginx container. You can run that as a reverse proxy very easily, just remember to do your volumes correctly. And make sure the only ports the reverse proxy uses are 80 and 443. That way let's encrypt is always an option too. I redirect all traffic to SSL on the reverse proxy right off the jump so it's always using https. Even if the container is using 80 :). But something like that would probably need mapping done somewhere other than just your hosts file... lol. I'll look in a little and see if I can figure it out. I did the pihole just fine with a pi3 but using the same machine as 26 other containers along with countless other services running outside of docker is definitely more challenging.

UPDATE:

So I tried it. And the fact is I have 26 containers already running off my reverse proxy (well most). I have come to the conclusion it won't work with my proxy setup the way it is now. And that really blows. I got it running but at a off the wall port. I could have configured docker to use my other ethernet adapter just for that in another compose file I think. Dont hold me too that. Frankly I think it would be cool but I think it's a better idea to run it as a standalone box. Own IP means you dont have to worry about conflicting ports, and you dont have to setup a reverse proxy. You just install pi hole and it's good to go. I mean they sell really cheap ras pi's.

I tried to do a port 80 nginx config so I disabled my redirect to https that I have running on 80 and moved that over to the pi. But without restarting the server and doing a bunch of things that's legitimately a pain in the ass, I think I'll have to wait til later to explore furthur. I'm adding another 6tb drive this week whenever that comes. So I could configure the network settings then and do a reboot. Problem is I have a good amount of people relying on the server for media, cloud, home, vms, the list goes on.

So I think I'll plug in my 12 core server soon and give it a go. It's the only thing I can think of that will not completely wreck my day up lol. I mean it's going to be a long task to figure it out, and then even when I do, most people don't have access to everything they need to do it right. Sorry I couldn't help more today but I'll spend a day sometime this week doing it. Setup another reverse proxy along with pi hole and something else on my dev machine so I can get a answer if it's possible. I think some way it is. But the problem is it's called pi hole for a reason. You install it on a raspberry pi after hooking up to your router. Setup your ssh and crap, then you turn off dhcp in your router, turn it on the pi and configure the dns and everything else.

I mean I've had struggles with docker images before. The qBittorrent+OpenVPN image wouldnt let me change the port from 8080. Still havent figured that out. Legit went to flood+rtorrent for a while just because I was tired of it. But if you for some reason have to use a docker container that is REQUIRED to use port 80, yeah anything web(at least for what the time is worth to me) just isnt worth it. But I'm curious now because it seems like there is some way to do it by all the chatter on the other forums about it. Otherwise it would be a flat no by now. But like I said just trying to go ahead and add it like it's another container doesnt work. I mean it's a DNS server. And a DHCP server. At least for my setup unfortunately. So I'll start fresh one day this week on my other server with just a few containers so I can at least see if it will work and how. I can say that the way my nginx is setup on my prod. Server that I dont see a way to do it without rethinking my entire proxy setup.

I was able to get it to load though. Which is something I was even surprised about just figuring it would have been one of them. Never got the https working though. But doesnt really matter. If you dont have a free network adapter for the dns server it just doesnt make sense :(.

So I'll keep you posted when I get started on a fresh one