add base path setting

.env.example

@@ -10,3 +10,5 @@ COOKIE_SECURE=false
 BIND_ADDR=127.0.0.1
 # port exposed on the host (the container itself always listens on 8000)
 HOST_PORT=8888
+# serving under a sub path? e.g. /mtg (no trailing slash). Leave empty for a domain root.
+BASE_PATH=