Fix: Production docker image built without test gems

[dragon-dxw]

Backport of https://github.com/citizensadvice/corporate-api/pull/55

The CA Corporate API is headless, so it doesn't use node, which means it's likely that this doesn't yet correctly handle node modules.

Changes in this PR

Before this change our Dockerfile wasn't correctly creating Docker images. It was bundling gems from the test group despite expecting RAILS_ENV to be set as production by default. We don't want to be deploying live artefacts that have test dependencies and tooling installed. They are bigger and increase the surface area for complexity and attacks.

The meat of this change is centered around the Dockerfile. Instead of using RAILS_ENV to try and let the user convey what gems they expected, we switch to creating more explicit docker build stages and select them with the target field instead.

Testing

1. DOCKER=1 script/test should still run all the tests (and have the test gems to do so)
2. DOCKER=1 script/server should still start up a local development server and be accesible at localhost:3000

If you want to test this in production too you can temporarily copy this into your app directory, into a file called docker-compose.production.yml. Once there you can run docker-compose -f docker-compose.production.yml up to check it boots in the right environment and watch that the right gems were installed:

version: "3.8"
services:
  web:
    build:
      context: .
      target: production
    command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b '0.0.0.0'"
    ports:
      - "3000:3000"
    depends_on:
      - db
    env_file:
       - .env.development.local
    environment:
      # DONE: Update rails-template with application name
      DATABASE_URL: postgres://postgres:password@db:5432/corporate-api-production
      AUTOMATICALLY_FIX_LINTING: "true"
    volumes:
      - .:/srv/app
    tty: true
    stdin_open: true
    networks:
      - prod

  db:
    image: postgres
    volumes:
      - db-data:/var/lib/postgresql/data
    environment:
      POSTGRES_PASSWORD: password
      POSTGRES_HOST_AUTH_METHOD: trust
    networks:
      - prod

networks:
  prod:

volumes:
  db-data:

[tahb]

These changes worked well for CAEW and we should get this merged in!

[pezholio]

I'd like to get this in, but the tests are failing at the mo

[tahb]

I've checked with Dragon and I'm going to try and move this along.

[tahb]

@pezholio back in business with a green test suite 👍

[tahb]

@erbridge I completely forgot about this! I've actioned your feedback now.

[erbridge]

Looks sensible to me. Minor commit message issue, but otherwise 👍.

[dragon-dxw]

LGTM, I can't approve as the creator, but @tabh feel free to approve on my behalf if you're happy with it :)

[tahb]

Approving on behalf of Dragon who's the original author and therefore can't :)

[lozette]

Been trying to get the tests passing on this to no avail, sorry!

[dragon-dxw]

Converted to draft because this is no longer fit to be published, but someone might want to build on this work in future.

Also it'll silence the notifications I get for it...

[mec]

Given the state and age of this work, I am closing this PR.