fix: convert GitHub username to lowercase for Docker tags

Author: Lars-

.github/workflows/docker-publish.yml

@@ -28,6 +28,11 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - name: Convert repository owner to lowercase
+        id: lowercase
+        run: |
+          echo "owner=$(echo ${{ github.repository_owner }} | tr '[:upper:]' '[:lower:]')" >> $GITHUB_OUTPUT
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
@@ -43,7 +48,7 @@ jobs:
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ${{ env.REGISTRY }}/${{ github.repository_owner }}/${{ env.IMAGE_NAME }}
+          images: ${{ env.REGISTRY }}/${{ steps.lowercase.outputs.owner }}/${{ env.IMAGE_NAME }}
           tags: |
             type=ref,event=branch
             type=ref,event=pr
@@ -67,6 +72,6 @@ jobs:
         if: github.event_name != 'pull_request'
         uses: actions/attest-build-provenance@v1
         with:
-          subject-name: ${{ env.REGISTRY }}/${{ github.repository_owner }}/${{ env.IMAGE_NAME }}
+          subject-name: ${{ env.REGISTRY }}/${{ steps.lowercase.outputs.owner }}/${{ env.IMAGE_NAME }}
           subject-digest: ${{ steps.build.outputs.digest }}
           push-to-registry: true