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.

.gitea/workflows/ci.yml

@@ -37,6 +37,12 @@ jobs:
             exit 1
           fi
 
+      - name: Compute lowercase image name
+        id: meta
+        run: |
+          REPO=git.rambossek.at/$(echo "${{ gitea.repository }}" | tr '[:upper:]' '[:lower:]')
+          echo "image=$REPO" >> "$GITHUB_OUTPUT"
+
       - uses: docker/setup-buildx-action@v3
 
       - uses: docker/login-action@v3
@@ -52,5 +58,5 @@ jobs:
           build-args: |
             VERSION=${{ gitea.ref_name }}
           tags: |
-            git.rambossek.at/${{ gitea.repository }}:${{ gitea.ref_name }}
-            git.rambossek.at/${{ gitea.repository }}:latest
+            ${{ steps.meta.outputs.image }}:${{ gitea.ref_name }}
+            ${{ steps.meta.outputs.image }}:latest