Support multiple values for COPY_PLUGINS_SRC, COPY_MODS_SRC, and COPY_CONFIG_SRC (#3377)

examples/paper/paper-build-plugins/Dockerfile

@@ -0,0 +1,5 @@
+FROM itzg/minecraft-server
+
+ENV TYPE=PAPER
+
+COPY plugins/*.jar /plugins/

examples/paper/paper-build-plugins/docker-compose.yml

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+services:
+  mc:
+    build: .
+    environment:
+      EULA: "TRUE"
+    ports:
+    - 25565:25565
+    stdin_open: true
+    tty: true

examples/paper/paper-build-plugins/plugins/.gitignore

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+*.jar

examples/paper/paper-build-plugins/plugins/README.md

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+Download Bukkit/Spigot plugin jars, such as [WorldEdit](https://dev.bukkit.org/projects/worldedit/files) and place them here. At image build time the `COPY` step will place those jars in `/plugins`. At container startup, the contents of `/plugins` are sync'ed into `/data/plugins` for use with Bukkit/Spigot/Paper server types.