docs: added ATM9 example (#2571)

Author: itzg

Diff for: examples/.gitignore

@@ -1,3 +1,4 @@
 data/
 modpacks/
-.env
+.env
+downloads/

Diff for: examples/atm9/docker-compose.yml

@@ -0,0 +1,22 @@
+version: "3.8"
+
+services:
+  mc:
+    image: itzg/minecraft-server
+    ports:
+      - "25565:25565"
+    environment:
+      EULA: "true"
+      MOD_PLATFORM: AUTO_CURSEFORGE
+      # allocate from https://console.curseforge.com/ and set in .env file
+      CF_API_KEY: ${CF_API_KEY}
+      CF_PAGE_URL: https://www.curseforge.com/minecraft/modpacks/all-the-mods-9
+      # Optional: select a specific version/file
+      #CF_FILENAME_MATCHER: "0.2.34"
+      MEMORY: 4G
+    volumes:
+      - mc-data:/data
+      - ./downloads:/downloads
+
+volumes:
+  mc-data: {}