I made this account just to start sharing my experience moving pi-hole to my Synology docker.
Many of you may have seen http://tonylawrence.com/posts/unix/synology/free-your-synology-ports/ and this got me doing some research on macvlan with docker.
If you ssh into your Synology as a root user and use the following command:
docker network create -d macvlan /
-o parent=eth0 Pi-hole
This will create a macvlan with the ip forced as a single ip (192.168.0.5) to use as a network for your docker image.You can then create your pi-hole docker image all through the UI now as this network will show up in docker. No port forwarding or anything is needed for your docker image just select Pi-hole as your network and delete bridge in the network section of the interface and set the ServerIP.
Using an IP calculator you can configure this to a single ip in ip-range section of the command.
Now modifying this a little I am trying to get cloudflared DOH running in a docker image but being that I am using eth0 for both pi-hole and cloudflared I am unable to have eth0 talking to itself. I hopefully will have some time over the weekend to further my research on this subject.