fix(ci): use GitHub Actions vars syntax for DockerHub username in workflow

Author: obeone

.github/workflows/build-and-publish.yaml

@@ -39,7 +39,7 @@ jobs:
         uses: docker/login-action@v2
         with:
           registry: docker.io
-          username: $DOCKERHUB_USERNAME
+          username: ${{ vars.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       - name: Set Docker image tags and build args