Add composition file for Docker script

Algolia documentation: [Run the crawl from the Docker image](https://docsearch.algolia.com/docs/run-your-own/#run-the-crawl-from-the-docker-image)

Example command:

```
docker run -it --env-file=.env -e "CONFIG=$(cat /path/to/your/config.json | jq -r tostring)" algolia/docsearch-scraper
```