Document baseUrl generation in the troubleshoot section

[alangrainger]

Discussed in #84

^{Originally posted by Sonlis April 22, 2025}
Hello there,

First of all thanks for this amazing project.

I did set it up behind a reverse proxy to handle SSL certificates, and had the issue of thumbnails not loading due to

Mixed Content: The page at '<URL>' was loaded over HTTPS, but requested an insecure image '<URL>'. This request has been blocked; the content must be served over HTTPS.

And indeed, the thumbnail URL included the private IP address of the container as the host. I checked the code and it seems it's a known "feature" :D

immich-public-proxy/app/src/render.ts

Line 95 in b33485b

// This may cause issues with some reverse proxies, in which case just make sure

    // This may cause issues with some reverse proxies, in which case just make sure
    // that you're sending the correct `host` header to IPP.

I think it would make sense to add it to the troubleshoot section, or a section on its own "Setup behind a reverse proxy", to save some time to the next people setting up IIP, and may help others that are not so code savy and might rebuke the project altogether because of this issue.