My desire is to use forum.example.com/forum for the forum and forum.example.com for a HTML page.
I searched for a solution and found that I need to setup an extra nginx proxy in front of the Docker container.
I tried following the instructions above, but the result was not successful.
Yes, that is true for hosted customers. But if you read after this:
It describes how to do it on your own server. The admonitions that it is difficult, complicated, and not really recommended are all true, however. Most everyone now agrees that there is no SEO advantage to using a subfolder.