add docker spec env vars

[crowesn]

Fixes #95

Add db_test container and test environment variables for running specs in docker

Changes proposed in this pull request:

• Add db_test container to docker-compose.yml
• Add .env.test environment variables
• Update readme

[scherztc]

@crowesn It looks like the delay caught a conflicting file. Any chance you could resolve the conflict?

[crowesn]

@scherztc done!