Add a .dockerignore

[jameslamb]

We should have a .dockerignore file to control what ends up in the app container (https://codefresh.io/docker-tutorial/not-ignore-dockerignore/).

The initial contents of this file is left to whoever picks up this issue! At a minimum it should include .git/

[samwaterbury]

Working on this too now!

[jameslamb]

@samwaterbury I assigned this to you just so no one else takes it. I realized that there is already a .dockerignore (https://github.com/uptake/groundhog/blob/master/.dockerignore), but see if you can recommend any things that should be added!

At a minimum, I think we should ignore the clients (since this container is only for running the app).