Provide default env config for dev and test

[mkasberg]

We should make it as easy as possible for a new developer to set up a dev environment. To that end, we shouldn't require a bunch of env vars to be set if we don't have a good reason to. For development and test environments, we can provide a default username and password for postgres, and we can configure that default user/pass in our docker-compose file. I've set this up in such a way that any of it can still be overridden by environment variables if you choose to do so, but you don't have to -- it will work out of the box.

• Edit the ERB in config/database.yml to fallback to a default username and pass if the env var isn't set.
• Require env vars in production. We don't want to make it easy to mistakenly use config we've published in our open source code.
• Provide the same defaults in our docker-compose.yml

Fixes #108

[mkasberg]

Oh, I should update the README with this info. Let me amend my commit to do that.