Fix Docker issues

- Copy `.ruby-version` file to Docker build context
- always run `docker build` in CI, even when not deploying

.dockerignore

@@ -1,3 +1,7 @@
-**/.*
+.dockerignore
+.editorconfig
+.github/
+.gitignore
+.rubocop.yml
 deployment/
 state.json

.github/workflows/main.yml

@@ -29,6 +29,9 @@ jobs:
     - name: Run RuboCop
       run: bundle exec rubocop
 
+    - name: Build Docker image
+      run: docker build --build-arg RUBY_VERSION=$(cat .ruby-version) .
+
   deploy:
     needs: checks
     if: github.repository_owner == 'Homebrew' && github.event_name == 'push'

Dockerfile

@@ -5,7 +5,7 @@ FROM ruby:$RUBY_VERSION-alpine
 RUN apk add --no-cache --virtual .build-deps git build-base
 
 WORKDIR /app
-COPY Gemfile* ./
+COPY .ruby-version Gemfile* ./
 ENV BUNDLE_DEPLOYMENT=1
 RUN bundle install && apk del --no-cache .build-deps