Hi,
I believe the front-end (port 3000) currently must be run at the root and not under a subdirectory. Typically, that means setting up a new subdomain growthbook.example.com and pointing that to port 3000. For the API (port 3100), the easiest is to have an additional subdomain growthbook-api.example.com, but it also may be possible to run it under a subdirectory like growthbook.example.com/gb-api/
There are some docs at https://docs.growthbook.io/self-host/env that might help