Use consistent root for serving admin

[dkarnutsch]

Currently it makes a difference from which directory the server is called.

This works

➜  ~/ws/comet-starter/admin/server git:(main) ✗ APP_PORT=3001 node index.js       
Current working directory: /Users/danielkarnutsch/ws/comet-starter/admin/server
Admin app listening at http://localhost:3001

while this is not working:

➜  ~/ws/comet-starter/admin git:(main) ✗ APP_PORT=3001 node server/index.js
Current working directory: /Users/danielkarnutsch/ws/comet-starter/admin
node:fs:441
    return binding.readFileUtf8(path, stringToFlags(options.flag));
                   ^

Error: ENOENT: no such file or directory, open '../build/index.html'

With this PR the admin is always served relative to the location of server.js. Furthermore server.js was renamed to index.js to make it possible to run the server using node server instead of using node server/server.js.

See also #388 (comment)

[nsams]

yes, it is strange that the server serves cwd/../build

wouldn't it make more sense to serve cwd or cwd/build?

[johnnyomair]

wouldn't it make more sense to serve cwd or cwd/build?

I'd also do this.

[dkarnutsch]

@johnnyomair @nsams done.

Now npm run serve (called from ./admin) and node server (called from ./admin) works.

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

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