chore(docker): improve env var setup

* Set `GITHUB_TOKEN` based on `GH_TOKEN`
* Enable [`docker buildx bake`][0] as the default engine to build images

[0]: https://docs.docker.com/build/bake/

docker-compose.yml

@@ -14,7 +14,9 @@ services:
     command: infinity
     environment:
       ANTHROPIC_API_KEY: ${NVIM_ANTHROPIC_API_KEY:-no-api}
-      GH_TOKEN: ${NVIM_GH_TOKEN:-no-token}
+      GH_TOKEN: &github_token ${NVIM_GH_TOKEN:-no-token}
+      GITHUB_TOKEN: *github_token
+      COMPOSE_BAKE: 'true'
     volumes:
       - './config/atuin:/home/coder/.config/atuin'
       - './config/git:/home/coder/.config/git'