Update GitHub Actions workflow to use Docker as the runner environment and fix secret variable references

.forgejo/workflows/push.yaml

@@ -5,15 +5,17 @@ on:
 
 jobs:
   build:
-    runs-on: catthehacker
+    runs-on: docker
+    container:
+      image: ghcr.io/catthehacker/ubuntu:act-latest
     steps:
       -
         name: Login to Docker Hub
         uses: docker/login-action@v3
         with:
           registry: code.edgarpierre.fr
-          username: ${{ secrets.DOCKER_USERNAME }}
-          password: ${{ secrets.DOCKER_PASSWORD }}
+          username: ${{ vars.DOCKER_PUSH_USERNAME }}
+          password: ${{ secrets.DOCKER_PUSH_PASSWORD }}
       -
         name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3