Better support for Mode-S Beast serial output

[EvenAR]

First of all, thank you for creating this amazing image - it makes sharing data incredibly easy!

I have a Mode-S Beast connected to a Raspberry Pi 4 via USB, and my current setup for using it with adsb.im involves the following steps:

1. Adding the following config to /opt/adsb/config/docker-compose.yml. This gives the ultrafeeder container access to the Mode-S Beast serial output:

   services:
     ultrafeeder:
       devices:
         - /dev/ttyUSB0:/dev/ttyUSB0
   

2. Adding the following environment variable on the "Expert" settings page. Ultrafeeder will then connect to the serial port /dev/ttyUSB0:

   READSB_DEVICE_TYPE=modesbeast
   

.. which took some time to figure out. What are your thoughts on adding this to the default docker-compose.yml? It would be even better if "Mode-S Beast over USB" were an option during initial setup :)

Best regards, Even