Add missing flags.yml download (#22181)

* Add missing flags.yml download

If this option is not present, then `docker compose up -d` error with:

    Error response from daemon: invalid mount config for type "bind": bind source path does not exist: /root/airbyte/flags.yml

* flags.yaml -> flags.yml

docs/deploying-airbyte/on-digitalocean-droplet.md

@@ -43,7 +43,7 @@ To install and start Airbyte :
 
 ```bash
   mkdir airbyte && cd airbyte
-  wget https://raw.githubusercontent.com/airbytehq/airbyte/master/{.env,docker-compose.yaml}
+  wget https://raw.githubusercontent.com/airbytehq/airbyte/master/{.env,flags.yml,docker-compose.yaml}
   docker compose up -d
 ```