[CI] Run rubocop and add rubocop-todo

There's no point in building the Dockerfile just for rubocop, but
doing it this way allows for easy integration of a test action in the future.

.github/prepare-docker-image/action.yml

@@ -0,0 +1,19 @@
+name: Load Docker Image
+description: Load Docker Image
+
+runs:
+  using: composite
+  steps:
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v2
+
+    - name: Build
+      uses: docker/build-push-action@v4
+      with:
+        cache-from: type=gha
+        load: true
+        tags: e621:test
+
+    - name: Set docker run command
+      run: echo "DOCKER_RUN=docker run --rm --network host -e POSTGRES_HOST=localhost -e RAILS_ENV=test -e CI=true -v $PWD:/app e621:test" >> "$GITHUB_ENV"
+      shell: bash

.github/workflows/checks.yml

@@ -0,0 +1,38 @@
+name: Checks
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+permissions:
+  contents: read
+
+jobs:
+  docker-build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: Build
+        uses: docker/build-push-action@v4
+        with:
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          tags: e621:test
+
+  rubocop:
+    runs-on: ubuntu-latest
+    needs: docker-build
+
+    steps:
+      - uses: actions/checkout@v3
+      - uses: ./.github/prepare-docker-image
+
+      - name: Run Rubocop
+        run: $DOCKER_RUN bundle exec rubocop --format github

.rubocop.yml

@@ -1,3 +1,10 @@
+<% if ENV["CI"] %>
+inherit_from: .rubocop_todo.yml
+inherit_mode:
+  merge:
+    - Exclude
+<% end %>
+
 require:
   - rubocop-rails
 
@@ -34,6 +41,8 @@ Metrics/BlockLength:
     - concerning
     - context
     - create_table
+    - factory
+    - FactoryBot.define
     - should
   Exclude:
     - config/routes.rb
@@ -73,7 +82,8 @@ Rails/InverseOf:
 
 Rails/Output:
   Exclude:
-    - db/*.rb
+    - db/seeds.rb
+    - db/fixes/*.rb
 
 Rails/ReversibleMigration:
   Enabled: false

README.md

@@ -44,7 +44,7 @@ Try this:
 
 `docker compose run --rm tests` to execute the test suite.
 
-`docker compose run --rm rubocop` to run the linter. Run it against changed files only, there are too many existing violations at the moment.
+`docker compose run --rm rubocop` to run the linter. You will see a truckload of violations but CI will ignore these.
 
 The postgres server accepts outside connections which you can use to access it with a local client. Use `localhost:34517` to connect to a database named `danbooru2` with the user `danbooru`. Leave the password blank, anything will work.