Add image configuration entry to new ci config

lib/gitlab/ci/config/node/image.rb

+module Gitlab
+  module Ci
+    class Config
+      module Node
+        ##
+        # Entry that represents a Docker image.
+        #
+        class Image < Entry
+          include Validatable
+
+          validations do
+            validates :config, type: String
+          end
+
+          def value
+            @config
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/ci/config/node/image_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Ci::Config::Node::Image do
+  let(:entry) { described_class.new(config) }
+
+  describe 'validation' do
+    context 'when entry config value is correct' do
+      let(:config) { 'ruby:2.2' }
+
+      describe '#value' do
+        it 'returns image string' do
+          expect(entry.value).to eq 'ruby:2.2'
+        end
+      end
+
+      describe '#errors' do
+        it 'does not append errors' do
+          expect(entry.errors).to be_empty
+        end
+      end
+
+      describe '#valid?' do
+        it 'is valid' do
+          expect(entry).to be_valid
+        end
+      end
+    end
+
+    context 'when entry value is not correct' do
+      let(:config) { ['ruby:2.2'] }
+
+      describe '#errors' do
+        it 'saves errors' do
+          expect(entry.errors)
+            .to include 'Image config should be a string'
+        end
+      end
+
+      describe '#valid?' do
+        it 'is not valid' do
+          expect(entry).not_to be_valid
+        end
+      end
+    end
+  end
+end