Publish the docker image

The instructions state:

```
docker run -d --name cocoindex-code \
  --volume "$(pwd):/workspace" \
  --volume cocoindex-db:/db \
  --volume cocoindex-model-cache:/root/.cache \
  ghcr.io/cocoindex-io/cocoindex-code:latest
```

When I run the command I get `Unable to find image 'ghcr.io/cocoindex-io/cocoindex-code:latest' locally`.