fix(ci): add Docker Hub login to avoid rate limit on base image pulls

The build-and-push job pulls node:20-alpine and nginx:stable-alpine from
Docker Hub during docker build. Without authentication these pulls hit
the unauthenticated rate limit, causing intermittent build failures.

Closes #8

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci.yml

@@ -51,6 +51,12 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
       - name: Log in to GHCR
         if: github.event_name == 'push' && github.ref == 'refs/heads/main'
         uses: docker/login-action@v3