Allow multiple knock interfaces

Nobody has claimed this yet.

### Enhancement Type

Improve an existing feature

### Describe the enhancement

I'm running my minecraft container in a gluetun network with a port-forward configured on my VPN. The gluetun network provides two network interfaces: eth0 and tun0. Both can be used depending on if I want to connect locally or externally through the VPN, so both need to respond to knocks.

It would be nice if `AUTOPAUSE_KNOCK_INTERFACE` supported a comma separated list of interfaces. This currently results in the following error being logged:
```
Selected interface "eth0, tun0" does not exist!
```

A docker compose configuration with gluetun looks something like this:
```yaml
version: '3.5'

services:
gluetun:
image: qmcgaw/gluetun
restart: unless-stopped
cap_add:
- NET_ADMIN
environment:
- VPN_SERVICE_PROVIDER=mullvad
- VPN_TYPE=wireguard
- WIREGUARD_PRIVATE_KEY=[VPN Provided Private Key]
- WIREGUARD_ADDRESSES=10.68.0.124/32
- SERVER_CITIES=Brussels
- FIREWALL_VPN_INPUT_PORTS=[VPN Provided Port]
ports:
- 25565:12345
minecraft:
image: itzg/minecraft-server
restart: unless-stopped
environment:
EULA=TRUE
SERVER_PORT=[VPN Provided Port]
AUTOPAUSE_KNOCK_INTERFACE=eth0,tun0 # Proposed syntax
tty: true
stdin_open: true
network_mode: "service:gluetun"
```

Start by tracing how AUTOPAUSE_KNOCK_INTERFACE is read and how the selected interface is validated in the container's knock handling. Use the provided eth0,tun0 configuration as the target case; done means both interfaces can respond to knocks without the current nonexistent-interface error.

Written by the indexing model from the issue text.