Improve docker-compose instructions

[mooreds]

You can 'merge' docker compose files.

So if we had a base docker compose file and then wanted to add kickstart and mailcatcher, we could have these three files:

• docker-compose.yml
• docker-compose-kickstart.yml
• docker-compose-mailcatcher.yml

Then we'd start up docker-compose via

 docker compose -f docker-compose.yml -f docker-compose-mailcatcher.yml -f docker-compose-kickstart.yml up

And all the files will be merged into one logical file.

More examples here https://github.com/spikex/rails-on-docker

and more here: https://docs.docker.com/compose/extends/

This will be useful to simplify this section: https://fusionauth.io/docs/v1/tech/installation-guide/docker#other-services