Update docs

Author: alangrainger

docs/installation.md

@@ -32,7 +32,7 @@ you.
 
 > [!WARNING]
 > If you're using Cloudflare, please make sure to set your `/share/video/*` path to Bypass Cache, otherwise you may
-> run into video playback issues. See [Troubleshooting](#troubleshooting) for more information.
+> run into video playback issues. See [Troubleshooting](/troubleshooting) for more information.
 
 ### Running alongside Immich on a single domain
 

docs/troubleshooting.md

@@ -10,6 +10,23 @@ set to bypass cache. I ran into this issue myself, and found
 
 I use Linux/Android, so this project is tested with BrowserStack for Apple/Windows devices.
 
+## Link previews show `http://` or a private IP
+
+The gallery itself uses relative URLs, so it works behind any reverse proxy. The one place IPP needs a fully qualified
+URL is the `og:image` tag that messaging apps use for link previews. Without `PUBLIC_BASE_URL`, IPP builds it from the
+incoming request, and behind a TLS-terminating reverse proxy that request arrives as plain `http://` on whatever
+hostname or IP the proxy used.
+
+Set `PUBLIC_BASE_URL` in your `docker-compose.yml` to the public address of IPP, without a trailing slash:
+
+```yaml
+environment:
+  PUBLIC_BASE_URL: https://your-proxy-url.com
+```
+
+If you serve IPP from several domains, leave it unset and make sure your reverse proxy forwards the original `Host`
+header. See [Running on a single domain](/running-on-single-domain).
+
 ## Can't reach Immich using `localhost:2283`
 
 This is a normal Docker thing, nothing to do with IPP.