feat: use config in root app directory

BREAKING CHANGE: configuration files must now be mounted at
`/opt/arena/index.json` instead of
`/opt/arena/src/server/config/index.json`. Failure to do so will result
in Arena starting with an empty config.

README.md

@@ -13,7 +13,7 @@ $ docker build -t <name-image> .
 To run a container, execute the following command. Note that we need to settle the location of `index.json` in this container via volume mounting:
 
 ```shell
-$ docker run -p 4567:4567 -v </local/route/to/index.json>:/opt/arena/node_modules/bull-arena/src/server/config/index.json <name-image>
+$ docker run -p 4567:4567 -v </local/route/to/index.json>:/opt/arena/index.json <name-image>
 ```
 
 See [the docs][usage] for `index.json`.

index.js

@@ -1,3 +1,12 @@
 const Arena = require('bull-arena');
 
-Arena();
+let config;
+try {
+  config = require('./index.json');
+} catch (err) {
+  if (err.code !== 'MODULE_NOT_FOUND') {
+    throw err;
+  }
+}
+
+Arena(config);