Merge branch '41342-add-ci-yml-example-for-browser-performance-testing-in-ce' into 'master'

Add CI YML example for Browser Performance Testing in CE

See merge request gitlab-org/gitlab-ce!16058

doc/ci/examples/browser_performance.md

+# Browser Performance Testing with the Sitespeed.io container
+
+This example shows how to run the [Sitespeed.io container](https://hub.docker.com/r/sitespeedio/sitespeed.io/) on your code by using
+GitLab CI/CD and [Sitespeed.io](https://www.sitespeed.io) using Docker-in-Docker.
+
+First, you need a GitLab Runner with the [docker-in-docker executor](../docker/using_docker_build.md#use-docker-in-docker-executor).
+
+Once you set up the Runner, add a new job to `.gitlab-ci.yml`, called `performance`:
+
+```yaml
+  stage: performance
+  image: docker:git
+  services:
+    - docker:dind
+  script:
+    - mkdir sitespeed-results
+    - docker run --shm-size=1g --rm -v "$(pwd)":/sitespeed.io sitespeedio/sitespeed.io --outputFolder sitespeed-results https://my.website.com
+  artifacts:
+    paths:
+    - sitespeed-results/
+```
+
+This will create a `performance` job in your CI/CD pipeline and will run Sitespeed.io against the webpage you define. The full HTML Sitespeed.io report will be saved as an artifact, and if you have Pages enabled it can be viewed directly in your browser. For further customization options of Sitespeed.io, including the ability to provide a list of URLs to test, please consult their [documentation](https://www.sitespeed.io/documentation/sitespeed.io/configuration/).
+
+For GitLab [Enterprise Edition Premium](https://about.gitlab.com/gitlab-ee/) users, a performance score can be automatically
+extracted and shown right in the merge request widget. Learn more about [Browser Performance Testing](https://docs.gitlab.com/ee/user/project/merge_requests/browser_performance_testing.html).
+
+## Performance testing on Review Apps
+
+The above CI YML is great for testing against static environments, and it can be extended for dynamic environments. There are a few extra steps to take to set this up:
+1. The `performance` job should run after the environment has started.
+1. In the `deploy` job, persist the hostname so it is available to the `performance` job. The same can be done for static environments like staging and production to unify the code path. Saving it as an artifact is as simple as `echo $CI_ENVIRONMENT_URL > environment_url.txt`.
+1. In the `performance` job read the artifact into an environment variable, like `$CI_ENVIRONMENT_URL`, and use it to parameterize the test URL's.
+1. Now you can run the Sitespeed.io container against the desired hostname and paths.
+
+A simple `performance` job would look like:
+
+```yaml
+  stage: performance
+  image: docker:git
+  services:
+    - docker:dind
+  script:
+    - export CI_ENVIRONMENT_URL=$(cat environment_url.txt)
+    - mkdir sitespeed-results
+    - docker run --shm-size=1g --rm -v "$(pwd)":/sitespeed.io sitespeedio/sitespeed.io --outputFolder sitespeed-results $CI_ENVIRONMENT_URL
+  artifacts:
+    paths:
+    - sitespeed-results/
+```