Add xdebug

[jlyon]

Not sure if this is absolutely appropriate, but bundling xdebug and xhprof would streamline local dev work.

[jonpugh]

I think you are right, but we probably would want to keep a separate "development" app container.

We wouldn't want to override this for the entire app, either... I'm thinking we should add some kind of environment-specific docker-compose overrides here, like so:

docker-compose:
  environment-overrides:
    # This would only override the docker compose for an environment named "dev"
    dev:
      app:
        image: terra/drupal-dev

What do you think, @jlyon ?

[jlyon]

@jonpugh This sounds like the right approach.

[jlyon]

@aschmoe