Unfortunately I've tried looking in Luanti's source code to see where it says this... (I'll have to grep a clone to maybe find it)
@@ -6,6 +6,7 @@ services:
ports:
- "30000:30000/udp"
volumes:
+ - ./data/mod_data:/mintest/mod_data
- ./data/minetest.conf:/minetest/minetest.conf
- ./data/worlds:/minetest/worlds
- ./data/games:/minetest/games