Update configuration for Heroku deployment

• Update docker-compose.yml to build from the local Dockerfile
• Update Dockerfile to set the copied files to be the same user/group as the user going to be running the service
• Update .dockerignore to ignore the .git folder, but no others. This keeps the package-lock.json file in the container, so npm install runs inside the container should just patch against whatever state the local node_modules folder is in.