CI template for installing cluster applications

Also, add docs on how to use this template to install GitLab managed
apps (v2)

changelogs/unreleased/ci_template_cluster_applications.yml

+---
+title: CI template for installing cluster applications
+merge_request: 20822
+author:
+type: added

doc/user/clusters/applications.md

@@ -429,6 +429,69 @@ administrator to run following command within a Rails console:
 Feature.enable(:enable_cluster_application_crossplane)
 ```
 
+## Install using GitLab CI (alpha)
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/merge_requests/20822) in GitLab 12.6.
+
+CAUTION: **Warning:**
+This is an _alpha_ feature, and it is subject to change at any time without
+prior notice.
+
+This alternative method allows users to install GitLab-managed
+applications using GitLab CI. It also allows customization of the
+install using Helm `values.yaml` files.
+
+Supported applications:
+
+- [Ingress](#install-ingress-using-gitlab-ci)
+
+### Usage
+
+To install applications using GitLab CI:
+
+1. Connect the cluster to a [cluster management project](management_project.md).
+1. In that project, add a `.gitlab-ci.yml` file with the following content:
+
+    ```yaml
+    include:
+      - template: Managed-Cluster-Applications.gitlab-ci.yml
+    ```
+
+1. Add a `.gitlab/managed-apps/config.yaml` file to define which
+  applications you would like to install. Define the `installed` key as
+  `true` to install the application and `false` to uninstall the
+  application. For example, to install Ingress:
+
+    ```yaml
+    ingress:
+      installed: true
+    ```
+
+1. Optionally, define `.gitlab/managed-apps/<application>/values.yaml` file to
+   customize values for the installed application.
+
+A GitLab CI pipeline will then run on the `master` branch to install the
+applications you have configured.
+
+### Install Ingress using GitLab CI
+
+To install ingress, define the `.gitlab/managed-apps/config.yaml` file
+with:
+
+```yaml
+ingress:
+  installed: true
+```
+
+Ingress will then be installed into the `gitlab-managed-apps` namespace
+of your cluster.
+
+You can customize the installation of Ingress by defining
+`.gitlab/managed-apps/ingress/values.yaml` file in your cluster
+management project. Refer to the
+[chart](https://github.com/helm/charts/tree/master/stable/nginx-ingress)
+for the available configuration options.
+
 ## Upgrading applications
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/24789) in GitLab 11.8.

lib/gitlab/ci/templates/Managed-Cluster-Applications.gitlab-ci.yml

+apply:
+  stage: deploy
+  image: "registry.gitlab.com/gitlab-org/cluster-integration/cluster-applications:v0.1.0"
+  environment:
+    name: production
+  variables:
+    TILLER_NAMESPACE: gitlab-managed-apps
+    GITLAB_MANAGED_APPS_FILE: $CI_PROJECT_DIR/.gitlab/managed-apps/config.yaml
+    INGRESS_VALUES_FILE: $CI_PROJECT_DIR/.gitlab/managed-apps/ingress/values.yaml
+  script:
+    - gitlab-managed-apps /usr/local/share/gitlab-managed-apps/helmfile.yaml
+  only:
+    refs:
+      - master

spec/lib/gitlab/ci/templates/managed_cluster_applications_gitlab_ci_yaml_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Managed-Cluster-Applications.gitlab-ci.yml' do
+  subject(:template) { Gitlab::Template::GitlabCiYmlTemplate.find('Managed-Cluster-Applications') }
+
+  describe 'the created pipeline' do
+    let_it_be(:user) { create(:user) }
+
+    let(:project) { create(:project, :custom_repo, namespace: user.namespace, files: { 'README.md' => '' }) }
+    let(:service) { Ci::CreatePipelineService.new(project, user, ref: pipeline_branch ) }
+    let(:pipeline) { service.execute!(:push) }
+    let(:build_names) { pipeline.builds.pluck(:name) }
+    let(:pipeline_branch) { 'master' }
+
+    before do
+      stub_ci_pipeline_yaml_file(template.content)
+    end
+
+    context 'for a default branch' do
+      it 'creates a apply job' do
+        expect(build_names).to match_array('apply')
+      end
+    end
+
+    context 'outside of default branch' do
+      let(:pipeline_branch) { 'a_branch' }
+
+      before do
+        project.repository.create_branch(pipeline_branch)
+      end
+
+      it 'has no jobs' do
+        expect { pipeline }.to raise_error(Ci::CreatePipelineService::CreateError, 'No stages / jobs for this pipeline.')
+      end
+    end
+  end
+end