Tweak CI workflow to match Ruby on Rails template

The only semantic change is switching the postgres image from
Debian-based to Alpine-based.

There are a couple of differences compared to the GitHub Actions Ruby
on Rails CI template:

 - The template only runs the job on push/pull-request for branch
   "master", but we run the job for all branches because we want to be
   able to push commits and have them tested without having to open a
   pull request.

 - The template runs on ubuntu-latest but we use ubuntu-20.04 because
   the build currently fails on ubuntu-latest.

 - The template doesn't have Docker health checks (for waiting until
   the services have started).

 - We have a "redis" service and a custom install script.

 - The template uses bin/rails etc., which we don't have yet.

 - The template has pretty step names, which we don't bother with.

 - The template's lint task uses bundler-audit, brakeman, and rubocop;
   we use standardrb.

.github/workflows/ci.yml

@@ -1,23 +1,24 @@
 name: Continuous Integration
 on: [push, pull_request]
 jobs:
-  build:
+  test:
     runs-on: ubuntu-20.04
     services:
       postgres:
-        image: postgres:11
+        image: postgres:11-alpine
+        ports: ["5432:5432"]
         env:
           POSTGRES_DB: nztraintest
+          POSTGRES_USER: postgres
           POSTGRES_PASSWORD: postgres
-        ports: ['5432:5432']
         options: >-
           --health-cmd "pg_isready -U postgres"
           --health-interval 10s
           --health-timeout 5s
           --health-retries 5
       redis:
         image: redis
-        ports: ['6379:6379']
+        ports: ["6379:6379"]
         options: >-
           --health-cmd "redis-cli ping"
           --health-interval 10s