forgot one dollar sign (#39)

Author: vlf-stefan-drobic

.github/workflows/docker-image.yml

@@ -10,7 +10,7 @@ jobs:
     permissions:
       contents: read
       packages: write
-      
+
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -29,7 +29,7 @@ jobs:
 
       - name: Set lowercase repository name
         run: |
-          echo "REPO_LOWER=${GITHUB_REPOSITORY,,}" >> GITHUB_ENV
+          echo "REPO_LOWER=${GITHUB_REPOSITORY,,}" >> $GITHUB_ENV
 
       - name: Build and push Docker image
         id: docker_build