build: pass ALLOWED_HOSTS env var to app

This variable allows you to set which IPs the app may be accessed from. This is useful to make it accessible when testing on a LAN network with another device (e.g. a mobile phone). The list of allowed hosts is passed as a space-separated string in the environment variable (e.g. `ALLOWED_HOSTS="127.0.0.1 localhost 192.168.100.25"`)

docker-compose.yml

@@ -46,3 +46,4 @@ services:
       - EMAIL_PORT=${EMAIL_PORT-"none"}
       - EMAIL_BACKEND_CONSOLE=${EMAIL_BACKEND_CONSOLE:-True}
       - EMAIL_USE_TLS=${EMAIL_USE_TLS:-True}
+      - ALLOWED_HOSTS=${ALLOWED_HOSTS-}