fix: use environment variable for DockerHub username in workflow

Author: dani-beltran

.github/workflows/docker-publish.yml

@@ -31,14 +31,14 @@ jobs:
         if: github.event_name != 'pull_request'
         uses: docker/login-action@v3
         with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          username: ${{ env.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       - name: Extract metadata (tags, labels) for Docker
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ${{ secrets.DOCKERHUB_USERNAME }}/${{ env.IMAGE_NAME }}
+          images: ${{ env.DOCKERHUB_USERNAME }}/${{ env.IMAGE_NAME }}
           tags: |
             type=ref,event=branch
             type=ref,event=pr