Use .env file

[aldeed]

Most other projects in the Reaction ecosystem now use a .env file for ENV variables that need to be set. They include a committed .env.example file, which is copied to a gitignored .env before building.

Work

1. Update the installation instructions in the public docs to document how environment variables work in the API project.
2. Create a .env.example file and .env file. Add all variables that are currently in the environment section of docker-compose.yml
3. Add .env to .gitignore
4. Remove the environment section from docker-compose.yml and replace with:

   env_file:
     - ./.env
   

5. Add pre-build project hook that creates/updates the .env file, the same as the script in the other repos. Note, however, that it cannot be in bin/setup because of Meteor's eager loading. It should be in .reaction/scripts.