Add Gitea release workflow; pull image from registry by default

CI builds and pushes the comfyui-nvidia image to the Gitea container
registry on every v* tag, mirroring the figment release workflow. Compose
now references the registry image (with build context kept for local
iteration) and the docs reflect the pull-by-default flow.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SETUP.md

@@ -32,16 +32,27 @@ Edit `.env`:
 - `OPEN_WEBUI_SECRET_KEY` — generate with `openssl rand -hex 32`. Keep stable;
   rotating it logs every user out.
 
-## 3. Build and start
+## 3. Start the stack
 
 ```sh
-docker compose up -d --build
+docker compose up -d
 ```
 
-First build pulls the CUDA base, PyTorch wheels, and ComfyUI source — expect
-several minutes. `docker compose logs -f comfyui` should end with a line like
+This pulls the prebuilt `comfyui-nvidia` image from the Gitea registry (CI
+publishes it on every `v*` tag) plus the upstream Open WebUI image, then
+brings both up. `docker compose logs -f comfyui` should end with a line like
 `To see the GUI go to: http://0.0.0.0:8188`.
 
+To build the image locally instead (e.g. while iterating on the Dockerfile):
+
+```sh
+docker compose build comfyui
+docker compose up -d
+```
+
+First local build pulls the CUDA base, PyTorch wheels, and ComfyUI source —
+expect several minutes.
+
 Health-check both services:
 
 ```sh