IPV6 with the same IPV4 port using `SERVER_PORT_V6`

Hi,

I'm trying hard to use this docker with IPV6 but really can't... IPV4 is working normally when I delete the AAAA IPV6 record from the DNS. I tested with netcat for android and ports are open and visible with IPV6 through a 4G external connection.

This is the command I'm using:
```
export PORT=19132; docker run --name=mc$PORT --restart=always -d -it -e EULA=TRUE -e SERVER_PORT_V6=$PORT -e SERVER_NAME=$PORT -e ALLOW_CHEATS=true -e DIFFICULTY=easy -e MODE=survival -e VERSION=latest -e SEED=4515655456848273743 -p $PORT:19132/udp -v mc-bedrock-data-$PORT:/data itzg/minecraft-bedrock-server
```

and docker ps output:
```
➜ braulio docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
faf0e7b2c263 itzg/minecraft-bedrock-server "/usr/local/bin/entr…" 4 minutes ago Up Less than a second (health: starting) 0.0.0.0:19132->19132/udp, [::]:19132->19132/udp mc19132
```

Start by reproducing the supplied docker run command with SERVER_PORT_V6 and inspect the resulting docker ps port bindings. Trace how the container handles SERVER_PORT_V6 and UDP publishing; done means the same port is reachable over both IPv4 and IPv6, or the limitation and required configuration are documented.

Written by the indexing model from the issue text.