Add SEARCH_BACKEND_PASSWORD env to archivebox container

Add the environment variable to make it explicit so that users are aware of the need to set the password in both containers.

docker-compose.yml

@@ -23,6 +23,7 @@ services:
             - SHOW_PROGRESS=False
             - SEARCH_BACKEND_ENGINE=sonic
             - SEARCH_BACKEND_HOST_NAME=sonic
+            - SEARCH_BACKEND_PASSWORD=SecretPassword
         volumes:
             - ./data:/data
         depends_on: