gtnh: support grabbing extra mods (#3788)

examples/gtnh/docker-compose-type-gtnh.yaml

@@ -15,6 +15,9 @@ services:
       # Use to prevent updates
       # SKIP_GTNH_UPDATE_CHECK: true
       MEMORY: 6G
+      # Bring in standard extra mods described at https://wiki.gtnewhorizons.com/wiki/Additional_Mods
+#      MODS: |
+#        https://github.com/Kynake/BetterFoliage/releases/download/1.2.1/BetterFoliage-LegacyEdition-1.2.1.jar
     volumes:
       # attach a managed volume, change to a relative or absolute host directory if needed
       - mc-data:/data

scripts/start-deployGTNH

@@ -276,4 +276,6 @@ fi
 # Start setup gtnh server files
 handleGTNH
 
+export USES_MODS=true
+
 exec "$(dirname "$0")/start-setupWorld" "$@"