Add Makefile for docker-compose dev commands

[kimadactyl]

Tasks that come to mind:

• [ ] Install
• [ ] Update / migrate
• [ ] Run tests
• [ ] Launch a server
• [ ] Create an admin user
• [ ] Twilio webhook command (don't really understand this one)

[wheresalice]

Install and update are really the same thing - build the image and run any migrations

I've added a Makefile with the following tasks:

• run - run the application in the foreground
• install - (re)-build the docker image and run any migrations
• test - run behave
• createadmin - create a superuser
• clean - shut everything down and remove volumes (doesn't remove the docker images)

I've not added one for the Twilio webhook command (though have renamed it). It's used for manually injecting mocked messages from Twilio into your local application. With decent test coverage I don't use this very often any more.