ci: standardize Docker Hub secret names to DOCKERHUB_USERNAME/DOCKERHUB_TOKEN

Author: GeiserX

.github/workflows/main.yml

@@ -10,7 +10,7 @@ on:
 
 env:
   REGISTRY: docker.io
-  IMAGE_NAME: ${{ secrets.DOCKER_USERNAME }}/website-diff
+  IMAGE_NAME: ${{ secrets.DOCKERHUB_USERNAME }}/website-diff
 
 jobs:
   test:
@@ -75,8 +75,8 @@ jobs:
       if: github.event_name != 'pull_request'
       uses: docker/login-action@v3
       with:
-        username: ${{ secrets.DOCKER_USERNAME }}
-        password: ${{ secrets.DOCKER_TOKEN }}
+        username: ${{ secrets.DOCKERHUB_USERNAME }}
+        password: ${{ secrets.DOCKERHUB_TOKEN }}
         registry: ${{ env.REGISTRY }}
 
     - name: Extract metadata