feat(single-compose.yml): add single-compose.yml for building leaner app container without meilisearch and mongodb services
- This is useful for deploying on Google, Azure, etc., as a single, leaner container.
- Instructions for running the container are added to the README.md file.
- The container requires a MongoDB Atlas connection string for the `MONGO_URI` environment variable.
- Remote Meilisearch may also be possible, but is not tested.
