workflows: revise `apidoc` workflow

.github/workflows/apidoc.yml

@@ -6,17 +6,19 @@ jobs:
   apidoc:
     runs-on: ubuntu-latest
     steps:
-      - uses: docker://ruby:latest
       - uses: Homebrew/actions/git-ssh@master
         with:
           git_user: BrewTestBot
           git_email: homebrew-test-bot@lists.sfconservancy.org
           key: ${{ secrets.RUBYDOC_DEPLOY_KEY }}
-      - run: |
-          # silence bundler complaining about being root
-          mkdir ~/.bundle
-          echo 'BUNDLE_SILENCE_ROOT_WARNING: "1"' > ~/.bundle/config
-          
+
+      - name: Set up Ruby
+        uses: actions/setup-ruby@v1
+        with:
+          version: '>=2.3'
+
+      - name: Set up repositories
+        run: |
           # clone rubydoc.brew.sh with SSH so we can push back
           git clone git@github.com:Homebrew/rubydoc.brew.sh
           cd rubydoc.brew.sh
@@ -24,9 +26,18 @@ jobs:
           # clone latest Homebrew/brew
           git clone --depth=1 https://github.com/Homebrew/brew
 
-          # run rake to build documentation
+      - name: Install RubyGems
+        run: |
+          # silence bundler complaining about being root
+          mkdir ~/.bundle
+          echo 'BUNDLE_SILENCE_ROOT_WARNING: "1"' > ~/.bundle/config
+
           gem install bundler
-          bundle install
+          bundle install --jobs 4 --retry 3
+
+      - name: Build and push API docs
+        run: |
+          # run rake to build documentation
           bundle exec rake
 
           # commit and push generated files