Use lowercase github organization name

Author: Michael Vasseur

.github/workflows/build-domjudge-container-PR.yml

@@ -65,10 +65,10 @@ jobs:
       - name: Build and push
         run: |
           for IMG in domserver judgehost default-judgehost-chroot; do
-            IMAGE_NAME="${{ github.repository_owner }}/$IMG:${{ env.DOMJUDGE_VERSION }}"
-            docker image tag "$IMAGE_NAME" ghcr.io/${{ github.repository_owner }}/$IMG:${{ env.PR_TAG }}
-            docker image tag "$IMAGE_NAME" ${{ github.repository_owner }}/$IMG:${{ env.PR_TAG }}
-            docker push ghcr.io/${{ github.repository_owner }}/$IMG:${{ env.PR_TAG }}
+            IMAGE_NAME="${GITHUB_REPOSITORY_OWNER@L}/$IMG:${{ env.DOMJUDGE_VERSION }}"
+            docker image tag "$IMAGE_NAME" ghcr.io/${GITHUB_REPOSITORY_OWNER@L}/$IMG:${{ env.PR_TAG }}
+            docker image tag "$IMAGE_NAME" ${GITHUB_REPOSITORY_OWNER@L}/$IMG:${{ env.PR_TAG }}
+            docker push ghcr.io/${GITHUB_REPOSITORY_OWNER@L}/$IMG:${{ env.PR_TAG }}
           done
 
       - name: Check for wrong permisions