Add prebuilt docker-compose, .env.example, and prebuilt install docs

README.md

@@ -47,7 +47,21 @@ These are separate and registered on **different** Entra apps. Don't conflate th
 
 Persistent state (profiles, settings, templates, logs, exports, certs) lives in `DataFolder`.
 
-## Installation — Docker
+## Installation — Docker (prebuilt image)
+
+Pulls the published image from the Gitea registry — no local build needed.
+
+```bash
+cp .env.example .env      # then edit .env with your OIDC values
+docker compose -f docker-compose.prebuilt.yml pull
+docker compose -f docker-compose.prebuilt.yml up -d
+```
+
+The compose file reads config from `.env` (see [`.env.example`](.env.example)). Pin a
+version with `SPTB_TAG`, e.g. `SPTB_TAG=v1.2.0` in `.env`. Don't quote values — the
+list form embeds literal quotes and breaks OIDC discovery.
+
+## Installation — Docker (build locally)
 
 ```bash
 docker compose up -d --build