feat(minecraft-server): onbuild trigger for downloading plugins and mods (#267)

* feat(minecraft-server): add onbuild trigger

Added a ONBUILD trigger for the [ToF-BuildTools](https://git.faldoria.de/tof/server/build-tools).
If a plugins.yml is provided the BuildTools will download all plugins configured in the YAML.

https://github.com/itzg/dockerfiles/issues/218

* feat(minecraft-server): add arg for build output

* fix(minecraft-server): syntax error in Dockerfile

* fix(minecraft-server): chown jar files after building

* feat(minecraft-server): cleanup buildtools after using

* fix(minecraft-server): remove /plugins volume

The /plugins volume causes weird behaviour when prepolulating it with
files on build. Removing it as an explicit volume fixes that.

* mc: add example Dockerfile for ToF builds

* docs(minecraft-server): add docs for new onbuild trigger

* fix(minecraft-server): use correct url for build tools download

minecraft-server/examples/ToF-build/Dockerfile

@@ -0,0 +1,3 @@
+FROM itzg/minecraft-server
+
+ENV TYPE=SPIGOT

minecraft-server/examples/ToF-build/plugins.yml

@@ -0,0 +1,13 @@
+plugins:
+  worldedit:
+    file: WorldEdit.jar
+    url: https://dev.bukkit.org/projects/worldedit/files/latest
+  FastAsyncWorldEdit:
+    file: FastAsyncWorldEdit.jar
+    url: https://empcraft.com/fawe/latest.php?bukkit
+  worldguard:
+    file: WorldGuard.jar
+    url: https://dev.bukkit.org/projects/worldguard/files/latest
+  citizens:
+    file: Citizens.jar
+    url: https://dev.bukkit.org/projects/citizens/files/latest