it’s the mac address of your rpi’s network interface or the mac address of a macvlan interface.
I doubt the docker engine interfere with the host’s ipv6 stack on its own. I don’t have a pi, but on my docker engine hosts, ipv6 doesn’t get removed.
Are you sure it’s not something you actually run on the host or inside a container that is responsible for this behavior? Did you test if the behavior is the same if you stop all containers? Furthermore, did you modify the configuration of the systemd service or /etc/docker/daemon.json?.
I would assume something like a second dhcp server that responds to the dhcp broadcast request of clients and bypasses the fritzbox dhcp server couuld prevent that the dns name is registered in the fritzbox dns server.
No need to share the container list. I not going to go through the list and read each image description to figure out what each container does or doesn’t do.
And are you even sure the fritzbox entry is even pointing to your rpi?
You didn’t mention whether you use maclan.
It is not uncommon that problems tend to hide in parts people don’t share with us, because they think it’s irrelevant.
You can find the reason if you start with an empty Docker host (no containers are running) and check the the name resolution and start to run containers one by one. After each container you check the name resolution and see what happens.
It culd be a container that changes something in your network. For example disables ipv6 temporarily, but a reboot resets those changes. I don’t recognize all of the software names you shared, but I could imagine that wg-easy (wireguard easy) changes kernel settings to disable ipv6. sysctl commands for example can change settings, but the configuration will not be persistent unless the same configs are saved in a config file. If the kernel settings are changed, you can export the kernel settings before running Docker:
sysctl -a > kernel-before-docker.txt
After Docker
sysctl -a > kernel-after-docker.txt
and after you start al the containers (if they don’t start with Docker)
sysctl -a > kernel-after-containers.txt
You could do the same after each container, but that is not necessary since nslookup can tell you when name resolution changes the diff can help you indentify which setting was changed
The ip addr command can also show you if ipv6 is disabled, because then you can’t find IPv6 address for the interfaces.
The reason could be something else, I just guessed, but you are the best person to find the actual reason. At least find out which container affects ipv6 addresses. We can try to help you with the rest.