ci: Switch backend to Docker Hub image (metados/blockscout:9.3.5)

Replace local build (doscan-backend:latest, pull_policy: never) with
Docker Hub image (metados/blockscout:9.3.5, pull_policy: always).
GHCR package is private due to org policy; Docker Hub is public.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JOY

docker-compose/docker-compose-mainnet.yml

@@ -63,8 +63,8 @@ services:
         condition: service_healthy
       redis-db:
         condition: service_healthy
-    image: doscan-backend:latest
-    pull_policy: never
+    image: metados/blockscout:9.3.5
+    pull_policy: always
     restart: always
     stop_grace_period: 5m
     container_name: backend