Possibility of saving a slave without necessity of having a mount point

Hi, I'm preparing a new remove slave server, and would like to run inside a cluster without a fixed volume, it would be very useful to have the possibility os passing my servers/domains list as an environment variable parameter, and in the startup the container would create the required `named.conf.default-zones` file
The docker-compose would be something like this (suggestions are welcome)

```
version: '3.5'

services:
bind9:
restart: unless-stopped
image: labbsr0x/dns-bind9
ports:
- "53:53/udp"
- "53:53/tcp"
environment:
MASTER_DOMAINS: 192.168.1.1:domain1.com,192.168.3.1:domain2.com
```

The startup script would then create the required file

If you agree, I could do this change and pull a patch

Thanks