Fallback to existing server file when getbukkit retrieval fails (#2721)

examples/spigot/docker-compose.yml

@@ -0,0 +1,13 @@
+services:
+  mc:
+    image: itzg/minecraft-server
+    environment:
+      EULA: true
+      TYPE: SPIGOT
+      VERSION: 1.20.4
+    ports:
+      - "25565:25565"
+    volumes:
+      - data:/data
+volumes:
+  data: {}