fix: force process.exit(0) after webpack build to prevent Node.js 20 hang

webpack 3.12.0 leaves open handles (internal timers/fs watchers) under
Node.js 20, causing the process to never exit naturally after a
successful build. The container hangs, docker run waits forever, and
the InfraBox job hits the 1-hour timeout.

The failure path already calls process.exit(1) explicitly; mirror that
for the success path with process.exit(0).

Root cause introduced by 34ade8f (node:8.9 -> node:20-alpine upgrade).

Author: Yuan Huang

src/dashboard-client/build/build.js

@@ -37,5 +37,6 @@ rm(path.join(config.build.assetsRoot, config.build.assetsSubDirectory), err => {
       '  Tip: built files are meant to be served over an HTTP server.\n' +
       '  Opening index.html over file:// won\'t work.\n'
     ))
+    process.exit(0)
   })
 })