Use the correct variables

Author: Michael Vasseur

.github/workflows/build-contributor-container-release.yml

@@ -23,8 +23,8 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: ghcr.io
-          username: ${{ env.DOCKERHUB_USERNAME }}
-          password: ${{ env.DOCKERHUB_TOKEN }}
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       - name: Build and push
         uses: docker/build-push-action@v5

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

@@ -27,8 +27,8 @@ jobs:
       - name: Login to Docker Hub
         uses: docker/login-action@v3
         with:
-          username: ${{ env.DOCKERHUB_USERNAME }}
-          password: ${{ env.DOCKERHUB_TOKEN }}
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
 
       - name: If needed overwrite the DOMJUDGE_VERSION for this run
         run: |