Update Docker Compose for GPT-3 Discord Bot

- Update `docker-compose.yml` to use the latest version of `ghcr.io/kav-k/gpt3discord:main` image
- Add `OPENAI_TOKEN` environment variable to `docker-compose.yml`

[docker-compose.yml]
- Change the `image` line in `docker-compose.yml` from `docker pull ghcr.io/kav-k/gpt3discord:main` to `ghcr.io/kav-k/gpt3discord:main`
- Add an `OPENAI_TOKEN` environment variable to `docker-compose.yml`

docker-compose.yml

@@ -1,7 +1,7 @@
 version: '3.8'
 services:
   gpt3discord:
-    image: docker pull ghcr.io/kav-k/gpt3discord:main
+    image: ghcr.io/kav-k/gpt3discord:main
     container_name: gpt3discord
     environment:
       OPENAI_TOKEN: "<openai_api_token>"