Add support for running rspec in local Docker development

[devjona]

What Issue Does This PR Cover, If Any?

Add support for running rspec in local Docker development

What Changed? And Why Did It Change?

Adds a test service to the docker-compose.dev.yml so that devs can:

# enter the container
docker exec -it skillrx-test-1 bash

# run specs:
/app# bin/rspec

• The app service runs in RAILS_ENV=development by default, thus it's nice to have a container for this app that runs in test.

How Has This Been Tested?

• it works locally