Create custom Github Actions for common steps (#27518)

.github/actions/setup-ruby/action.yml

@@ -0,0 +1,23 @@
+name: 'Setup RUby'
+description: 'Setup a Ruby environment ready to run the Mastodon code'
+inputs:
+  ruby-version:
+    description: The Ruby version to install
+    default: '.ruby-version'
+  additional-system-dependencies:
+    description: 'Additional packages to install'
+
+runs:
+  using: 'composite'
+  steps:
+    - name: Install system dependencies
+      shell: bash
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y libicu-dev libidn11-dev ${{ inputs.additional-system-dependencies }}
+
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{ inputs.ruby-version }}
+        bundler-cache: true