Switch base images from Docker Hub to GHCR mirror

Avoids Docker Hub 429 rate limits by pulling node:20-alpine and
nginx:stable-alpine from ghcr.io/cartsnitch/mirror/. GHCR login
now runs on all builds (not just main push) to authenticate pulls.

Ref: cartsnitch/infra#7, CAR-55

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

.github/workflows/ci.yml

@@ -52,7 +52,6 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Log in to GHCR
-        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}