Merge branch 'add-kaniko-docs' into 'master'

Initial kaniko documentation

See merge request gitlab-org/gitlab-ce!21216

doc/ci/docker/README.md

@@ -6,3 +6,4 @@ comments: false
 
 - [Using Docker Images](using_docker_images.md)
 - [Using Docker Build](using_docker_build.md)
+- [Using kaniko](using_kaniko.md)

doc/ci/docker/using_kaniko.md

+# Building images with kaniko and GitLab CI/CD
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/45512) in GitLab 11.2.
+Requires GitLab Runner 11.2 and above.
+
+[kaniko](https://github.com/GoogleContainerTools/kaniko) is a tool to build
+container images from a Dockerfile, inside a container or Kubernetes cluster.
+
+kaniko solves two problems with using the
+[docker-in-docker build](using_docker_build.md#use-docker-in-docker-executor) method:
+
+1. Docker-in-docker requires [privileged mode](https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities)
+   in order to function, which is a significant security concern.
+1. Docker-in-docker generally incurs a performance penalty and can be quite slow.
+
+## Requirements
+
+In order to utilize kaniko with GitLab, a [GitLab Runner](https://docs.gitlab.com/runner/)
+using either the [Kubernetes](https://docs.gitlab.com/runner/executors/kubernetes.html),
+[Docker](https://docs.gitlab.com/runner/executors/docker.html), or
+[Docker Machine](https://docs.gitlab.com/runner/executors/docker_machine.html)
+executors is required.
+
+## Building a Docker image with kaniko
+
+When building an image with kaniko and GitLab CI/CD, you should be aware of a
+few important details:
+
+- The kaniko debug image is recommended (`gcr.io/kaniko-project/executor:debug`)
+  because it has a shell, and a shell is required for an image to be used with
+  GitLab CI/CD.
+- The entrypoint will need to be [overridden](using_docker_images.md#overriding-the-entrypoint-of-an-image),
+  otherwise the build script will not run.
+- A Docker `config.json` file needs to be created with the authentication
+  information for the desired container registry.
+
+---
+
+In the following example, kaniko is used to build a Docker image and then push
+it to [GitLab Container Registry](../../user/project/container_registry.md).
+The job will run only when a tag is pushed. A `config.json` file is created under
+`/root/.docker` with the needed GitLab Container Registry credentials taken from the
+[environment variables](../variables/README.md#predefined-variables-environment-variables)
+GitLab CI/CD provides. In the last step, kaniko uses the `Dockerfile` under the
+root directory of the project, builds the Docker image and pushes it to the
+project's Container Registry while tagging it with the Git tag:
+
+```yaml
+build:
+  stage: build
+  image:
+    name: gcr.io/kaniko-project/executor:debug
+    entrypoint: [""]
+  script:
+    - mkdir -p /root/.docker
+    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /root/.docker/config.json
+    - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG
+  only:
+    - tags
+```