Fix errore Docker build quando secrets non configurati

- Aggiunto step 'Prepare image name' per gestire nome immagine dinamico
- Se secrets configurati: usa username/ha-voltronic-mqtt
- Se secrets mancanti: usa local/ha-voltronic-mqtt (build locale)
- Corretto flag SHOULD_PUSH basato su presenza secrets
- Risolto errore 'invalid tag' quando DOCKER_USERNAME è vuoto

Ora il workflow esegue build anche senza secrets configurati

.gitea/workflows/docker-build.yml

@@ -29,12 +29,23 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v2
 
+      - name: Prepare image name
+        id: image-name
+        run: |
+          if [ -n "${{ secrets.DOCKER_USERNAME }}" ]; then
+            echo "IMAGE_NAME=${{ secrets.DOCKER_USERNAME }}/ha-voltronic-mqtt" >> $GITHUB_OUTPUT
+            echo "SHOULD_PUSH=true" >> $GITHUB_OUTPUT
+          else
+            echo "IMAGE_NAME=local/ha-voltronic-mqtt" >> $GITHUB_OUTPUT
+            echo "SHOULD_PUSH=false" >> $GITHUB_OUTPUT
+          fi
+
       - name: Docker meta
         id: meta
         uses: docker/metadata-action@v4
         with:
           images: |
-            ${{ secrets.DOCKER_USERNAME }}/ha-voltronic-mqtt
+            ${{ steps.image-name.outputs.IMAGE_NAME }}
           tags: |
             type=ref,event=branch
             type=ref,event=pr
@@ -43,7 +54,7 @@ jobs:
             type=raw,value=latest,enable={{is_default_branch}}
 
       - name: Login to Docker Hub
-        if: github.event_name != 'pull_request' && secrets.DOCKER_USERNAME != ''
+        if: github.event_name != 'pull_request' && steps.image-name.outputs.SHOULD_PUSH == 'true'
         uses: docker/login-action@v2
         with:
           username: ${{ secrets.DOCKER_USERNAME }}
@@ -55,7 +66,7 @@ jobs:
           context: .
           file: ./Dockerfile.multiarch
           platforms: linux/arm/v6,linux/arm/v7,linux/arm64
-          push: ${{ github.event_name != 'pull_request' && secrets.DOCKER_USERNAME != '' }}
+          push: ${{ steps.image-name.outputs.SHOULD_PUSH == 'true' && github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           build-args: |