add PAPERLESS_URL env variable for CSRF

docker/compose/docker-compose.env

@@ -22,6 +22,10 @@
 # Docker setup does not use the configuration file.
 # A few commonly adjusted settings are provided below.
 
+# This is required if you will be exposing Paperless-ngx on a public domain
+# (if doing so please consider security measures such as reverse proxy)
+#PAPERLESS_URL=https://paperless.example.com
+
 # Adjust this key if you plan to make paperless available publicly. It should
 # be a very long sequence of random characters. You don't need to remember it.
 #PAPERLESS_SECRET_KEY=change-me