fix: remove prefix from command and set a working-dir to /app (#76)

In the past, this image used to pass --prefix /app to the npm start command, which is used to
force non-global commands to run in the specified folder. We are removing that prefix and instead
setting up a working-dir to keep consisdenty with upstream.

centraldashboard/rockcraft.yaml

@@ -15,8 +15,9 @@ services:
   serve:
     override: replace
     summary: "Kubeflow central dashboard service"
-    command: "/usr/bin/npm start --prefix /app"
+    command: "/usr/bin/npm start"
     startup: enabled
+    working-dir: "/app"
     environment:
       NODE_ENV: production