docker-compose scale doesn't exist at Docker 3

First of all, thanks for these Docker images & in particular the docker compose config. Makes it very easy to experiment with Citus.

From the top-level README.md:

> But you can add more workers at will using docker-compose scale in another tab. For instance, to bring your worker count to five…
> docker-compose -p citus scale worker=5

`docker-compose scale` [doesn't exist at Docker 3](https://docs.docker.com.xy2401.com/v17.09/compose/reference/scale/). I'd do a PR but I'm not a docker-compose expert. I successfully used the command below.

```docker-compose -p citus up --scale worker=2```