Have you configured dotCMS to look for 'index.html'?
By default dotCMS looks for a page simply named 'index' in the path you are trying to access. This property is controlled in dotmarketing-config.properties. The property name is CMS_INDEX_PAGE. Typically, if you can access the page by directly specifying index.html and not by accessing the root path then you need to change this configuration (or rename the file).
Regarding
5.2.8.4: This version is no longer in support and is not receiving security updates. We provide docker compose examples of standing up the latest version of dotCMS with images for the database and elasticsearch included in the compose. You can find those here:
https://www.dotcms.com/download#docker
Let me know if this helps!