Add prepare action for environments

It can be used to annotate jobs which don't deploy
anything, but need environment variables

changelogs/unreleased/208655-introduce-prepare-keyword-to-environment-action-to-annotate-non-de.yml

+---
+title: Introduce prepare environment action to annotate non-deployment jobs
+merge_request: 35642
+author:
+type: added

lib/gitlab/ci/config/entry/environment.rb

@@ -44,7 +44,7 @@ module Gitlab
 
               validates :action,
                         type: String,
-                        inclusion: { in: %w[start stop], message: 'should be start or stop' },
+                        inclusion: { in: %w[start stop prepare], message: 'should be start, stop or prepare' },
                         allow_nil: true
 
               validates :on_stop, type: String, allow_nil: true

spec/lib/gitlab/ci/config/entry/environment_spec.rb

@@ -102,6 +102,17 @@ RSpec.describe Gitlab::Ci::Config::Entry::Environment do
     end
   end
 
+  context 'when prepare action is used' do
+    let(:config) do
+      { name: 'production',
+        action: 'prepare' }
+    end
+
+    it 'is valid' do
+      expect(entry).to be_valid
+    end
+  end
+
   context 'when wrong action type is used' do
     let(:config) do
       { name: 'production',
@@ -137,7 +148,7 @@ RSpec.describe Gitlab::Ci::Config::Entry::Environment do
     describe '#errors' do
       it 'contains error about invalid action' do
         expect(entry.errors)
-          .to include 'environment action should be start or stop'
+          .to include 'environment action should be start, stop or prepare'
       end
     end
   end

spec/lib/gitlab/ci/pipeline/seed/deployment_spec.rb

@@ -102,6 +102,19 @@ RSpec.describe Gitlab::Ci::Pipeline::Seed::Deployment do
       end
     end
 
+    context 'when job has environment attribute with prepare action' do
+      let(:attributes) do
+        {
+          environment: 'production',
+          options: { environment: { name: 'production', action: 'prepare' } }
+        }
+      end
+
+      it 'returns nothing' do
+        is_expected.to be_nil
+      end
+    end
+
     context 'when job does not have environment attribute' do
       let(:attributes) { { name: 'test' } }