Automatic mac address

[GioF71]

If SQUEEZELITE_MAC_ADDRESS is not specified, the startup script will generate a new one and place it in /config/mac-address.txt
Just add a persistent volume to your container and the mac address will always stay the same, so you won't have to always setup audio properties, scrobbling, etc.

Use the new variable CONFIG_FILE_PREFIX to add a prefix to files in /config (currently only mac-address.txt) if you want to use the same directory for multiple containers.