CI: lowercase image repository name; compose example too

Docker registry names must be lowercase, so PUBLIC/gpu-turnstile was
rejected by buildx. The workflow now lowercases gitea.repository.

SPEC.md

@@ -194,7 +194,9 @@ are new.
      available in the runner image
   2. on a version tag only (`vX.Y.Z`, enforced): build the image with buildx
      and push it to the Gitea registry
-     `git.rambossek.at/<owner>/gpu-turnstile` tagged `:<tag>` and `:latest`.
+     `git.rambossek.at/<owner>/gpu-turnstile` tagged `:<tag>` and `:latest`
+     (the repository path is lowercased in the workflow; Docker registry
+     names must be lowercase).
      Login uses the repo secret `REGISTRY_TOKEN` (an access token with
      `write:package` scope) because the automatic `GITEA_TOKEN` cannot push
      packages; the username is just `gitea.actor`.
@@ -205,7 +207,7 @@ are new.
 
 ```yaml
   gpu-turnstile:
-    image: git.rambossek.at/<owner>/gpu-turnstile:v0.1.0
+    image: git.rambossek.at/<owner>/gpu-turnstile:v0.1.0  # owner lowercased, e.g. "public"
     environment:
       OLLAMA_URL: http://<workstation-ip>:11435
       COMFY_URL:  http://<workstation-ip>:8189