Custom React App does not load when running manager using `OR_CUSTOM_APP_DOCROOT`

[wborn]

While testing the Custom React App in IntelliJ IDEA I found that it does not load properly.
It might also not work well when deploying a custom project using this app to production.

To reproduce:

1. Configure OR_CUSTOM_APP_DOCROOT=deployment/build/image/manager/app as env var in the Application run configuration in IntelliJ IDEA
2. Build the code using: ./gradlew clean installDist
3. Start the manager by running the run configuration in IntelliJ IDEA
4. Open the App in your browser by navigating to: http://localhost:8080/custom-react/

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The React App does not show, it seems to load files using the / path instead of /custom-react/ see this console output:

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The App does work when running it via npm run serve