Add GitLab integration for scaffolder

This adds a GitLab integration for the scaffolder backend.

We're introduceing a preparer and a publisher for GitLab so that we can
read templates from GitLab and publish them to a configured GitLab
instance. The two instances don't need to be the same. For instance,
templates could be public on gitlab.com, but the created repos will live
in a hosted GitLab somewhere else.

The publisher gets its own config object in `app-config.yaml` where the
target instance and token can be specified.

The service catalogue defines both `gitlab` and `gitlab/api` as
processors. They are both handled by the same preparer.

Closes #2372

docs/features/software-templates/index.md

@@ -8,8 +8,8 @@ you create Components inside Backstage
 
 The Software Templates part of Backstage is a tool that can help you create
 Components inside Backstage. By default, it has the ability to load skeletons of
-code, template in some variables, and then publish the template to some location
-like GitHub.
+code, template in some variables, and then publish the template to some
+locations like GitHub or GitLab.
 
 <video width="100%" height="100%" controls>
   <source src="https://backstage.io/blog/assets/2020-08-05/feature.mp4" type="video/mp4">

docs/features/software-templates/installation.md

@@ -87,13 +87,17 @@ import {
   createRouter,
   FilePreparer,
   GithubPreparer,
+  GitlabPreparer,
   Preparers,
+  Publishers,
   GithubPublisher,
+  GitlabPublisher,
   CreateReactAppTemplater,
   Templaters,
   RepoVisilityOptions,
 } from '@backstage/plugin-scaffolder-backend';
 import { Octokit } from '@octokit/rest';
+import { Gitlab } from '@gitbeaker/node';
 import type { PluginEnvironment } from '../types';
 import Docker from 'dockerode';
 
@@ -104,17 +108,20 @@ export default async function createPlugin({
   const cookiecutterTemplater = new CookieCutter();
   const craTemplater = new CreateReactAppTemplater();
   const templaters = new Templaters();
-  // Register default templaters
   templaters.register('cookiecutter', cookiecutterTemplater);
   templaters.register('cra', craTemplater);
 
   const filePreparer = new FilePreparer();
   const githubPreparer = new GithubPreparer();
+  const gitlabPreparer = new GitlabPreparer(config);
   const preparers = new Preparers();
 
-  // Register default preparers
   preparers.register('file', filePreparer);
   preparers.register('github', githubPreparer);
+  preparers.register('gitlab', gitlabPreparer);
+  preparers.register('gitlab/api', gitlabPreparer);
+
+  const publishers = new Publishers();
 
   const githubToken = config.getString('scaffolder.github.token');
   const repoVisibility = config.getString(
@@ -122,17 +129,32 @@ export default async function createPlugin({
   ) as RepoVisilityOptions;
 
   const githubClient = new Octokit({ auth: githubToken });
-  const publisher = new GithubPublisher({
+  const githubPublisher = new GithubPublisher({
     client: githubClient,
     token: githubToken,
     repoVisibility,
   });
+  publishers.register('file', githubPublisher);
+  publishers.register('github', githubPublisher);
+
+  const gitLabConfig = config.getOptionalConfig('scaffolder.gitlab.api');
+
+  if (gitLabConfig) {
+    const gitLabToken = gitLabConfig.getString('token');
+    const gitLabClient = new Gitlab({
+      host: gitLabConfig.getOptionalString('baseUrl'),
+      token: gitLabToken,
+    });
+    const gitLabPublisher = new GitlabPublisher(gitLabClient, gitLabToken);
+    publishers.register('gitlab', gitLabPublisher);
+    publishers.register('gitlab/api', gitLabPublisher);
+  }
 
   const dockerClient = new Docker();
   return await createRouter({
     preparers,
     templaters,
-    publisher,
+    publishers,
     logger,
     dockerClient,
   });
@@ -179,7 +201,7 @@ catalog:
       target: https://github.com/spotify/cookiecutter-golang/blob/master/template.yaml
 ```
 
-### Runtime Dependencies
+### Runtime Dependencies / Configuration
 
 For the scaffolder backend plugin to function, it needs a GitHub access token,
 and access to a running Docker daemon. You can create a GitHub access token
@@ -189,10 +211,18 @@ docs on creating private GitHub access tokens is available
 Note that the need for private GitHub access tokens will be replaced with GitHub
 Apps integration further down the line.
 
+#### Github
+
 The Github access token is retrieved from environment variables via the config.
 The config file needs to specify what environment variable the token is
 retrieved from. Your config should have the following objects.
 
+#### Gitlab
+
+For Gitlab, we currently support the configuration of the GitLab publisher and
+allows to configure the private access token and the base URL of a GitLab
+instance:
+
 ```yaml
 scaffolder:
   github:
@@ -200,6 +230,12 @@ scaffolder:
       $secret:
         env: GITHUB_ACCESS_TOKEN
     visibility: public # or 'internal' or 'private'
+  gitlab:
+    api:
+      baseUrl: https://gitlab.com
+      token:
+        $secret:
+          env: SCAFFOLDER_GITLAB_PRIVATE_TOKEN
 ```
 
 You can configure who can see the new repositories that the scaffolder creates