Normalize IMAGE_NAMESPACE to lowercase

.github/workflows/build-images.yaml

@@ -23,7 +23,6 @@ on:
 
 env:
   REGISTRY: ghcr.io
-  IMAGE_NAMESPACE: ${{ github.repository }}
 
 jobs:
   build:
@@ -35,6 +34,9 @@ jobs:
       fail-fast: false
 
     steps:
+      - name: Set lowercase image namespace
+        run: echo "IMAGE_NAMESPACE=${GITHUB_REPOSITORY,,}" >> $GITHUB_ENV
+
       - name: Checkout repository
         uses: actions/checkout@v4