# Building Docker images with GitLab CI/CD

GitLab CI/CD allows you to use Docker Engine to build and test docker-based projects.

TIP: **Tip:**
This also allows to you to use `docker-compose` and other docker-enabled tools.

One of the new trends in Continuous Integration/Deployment is to:

1. Create an application image
1. Run tests against the created image
1. Push image to a remote registry
1. Deploy to a server from the pushed image

It's also useful when your application already has the `Dockerfile` that can be
used to create and test an image:

```bash
docker build -t my-image dockerfiles/
docker run my-docker-image /script/to/run/tests
docker tag my-image my-registry:5000/my-image
docker push my-registry:5000/my-image
```

This requires special configuration of GitLab Runner to enable `docker` support
during jobs.

## Runner Configuration

There are three methods to enable the use of `docker build` and `docker run` during jobs; each with their own tradeoffs.

### Use shell executor

The simplest approach is to install GitLab Runner in `shell` execution mode.
GitLab Runner then executes job scripts as the `gitlab-runner` user.

1. Install [GitLab Runner](https://gitlab.com/gitlab-org/gitlab-runner/#installation).

1. During GitLab Runner installation select `shell` as method of executing job scripts or use command:

    ```bash
    sudo gitlab-runner register -n \
      --url https://gitlab.com/ \
      --registration-token REGISTRATION_TOKEN \
      --executor shell \
      --description "My Runner"
    ```

2. Install Docker Engine on server.

    For more information how to install Docker Engine on different systems
    checkout the [Supported installations](https://docs.docker.com/engine/installation/).

3. Add `gitlab-runner` user to `docker` group:

    ```bash
    sudo usermod -aG docker gitlab-runner
    ```

4. Verify that `gitlab-runner` has access to Docker:

    ```bash
    sudo -u gitlab-runner -H docker info
    ```

    You can now verify that everything works by adding `docker info` to `.gitlab-ci.yml`:

    ```yaml
    before_script:
      - docker info

    build_image:
      script:
        - docker build -t my-docker-image .
        - docker run my-docker-image /script/to/run/tests
    ```

5. You can now use `docker` command and install `docker-compose` if needed.

NOTE: **Note:**
By adding `gitlab-runner` to the `docker` group you are effectively granting `gitlab-runner` full root permissions.
For more information please read [On Docker security: `docker` group considered harmful](https://www.andreas-jung.com/contents/on-docker-security-docker-group-considered-harmful).

### Use docker-in-docker executor

The second approach is to use the special docker-in-docker (dind)
[Docker image](https://hub.docker.com/_/docker/) with all tools installed
(`docker` and `docker-compose`) and run the job script in context of that
image in privileged mode.

In order to do that, follow the steps:

1. Install [GitLab Runner](https://docs.gitlab.com/runner/install).

1. Register GitLab Runner from the command line to use `docker` and `privileged`
   mode:

    ```bash
    sudo gitlab-runner register -n \
      --url https://gitlab.com/ \
      --registration-token REGISTRATION_TOKEN \
      --executor docker \
      --description "My Docker Runner" \
      --docker-image "docker:stable" \
      --docker-privileged
    ```

    The above command will register a new Runner to use the special
    `docker:stable` image which is provided by Docker. **Notice that it's using
    the `privileged` mode to start the build and service containers.** If you
    want to use [docker-in-docker] mode, you always have to use `privileged = true`
    in your Docker containers.

    The above command will create a `config.toml` entry similar to this:

    ```
    [[runners]]
      url = "https://gitlab.com/"
      token = TOKEN
      executor = "docker"
      [runners.docker]
        tls_verify = false
        image = "docker:stable"
        privileged = true
        disable_cache = false
        volumes = ["/cache"]
      [runners.cache]
        Insecure = false
    ```

1. You can now use `docker` in the build script (note the inclusion of the
   `docker:dind` service):

    ```yaml
    image: docker:stable

    variables:
      # When using dind service we need to instruct docker, to talk with the
      # daemon started inside of the service. The daemon is available with
      # a network connection instead of the default /var/run/docker.sock socket.
      #
      # The 'docker' hostname is the alias of the service container as described at
      # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services
      #
      # Note that if you're using Kubernetes executor, the variable should be set to
      # tcp://localhost:2375 because of how Kubernetes executor connects services
      # to the job container
      DOCKER_HOST: tcp://docker:2375/
      # When using dind, it's wise to use the overlayfs driver for
      # improved performance.
      DOCKER_DRIVER: overlay2

    services:
    - docker:dind

    before_script:
    - docker info

    build:
      stage: build
      script:
      - docker build -t my-docker-image .
      - docker run my-docker-image /script/to/run/tests
    ```

Docker-in-Docker works well, and is the recommended configuration, but it is
not without its own challenges:

- By enabling `--docker-privileged`, you are effectively disabling all of
  the security mechanisms of containers and exposing your host to privilege
  escalation which can lead to container breakout. For more information, check
  out the official Docker documentation on
  [Runtime privilege and Linux capabilities][docker-cap].
- When using docker-in-docker, each job is in a clean environment without the past
  history. Concurrent jobs work fine because every build gets it's own
  instance of Docker engine so they won't conflict with each other. But this
  also means jobs can be slower because there's no caching of layers.
- By default, `docker:dind` uses `--storage-driver vfs` which is the slowest
  form offered. To use a different driver, see
  [Using the overlayfs driver](#using-the-overlayfs-driver).
- Since the `docker:dind` container and the runner container don't share their
  root filesystem, the job's working directory can be used as a mount point for
  children containers. For example, if you have files you want to share with a
  child container, you may create a subdirectory under `/builds/$CI_PROJECT_PATH`
  and use it as your mount point (for a more thorough explanation, check [issue
  #41227](https://gitlab.com/gitlab-org/gitlab-ce/issues/41227)):

    ```yaml
    variables:
      MOUNT_POINT: /builds/$CI_PROJECT_PATH/mnt

    script:
      - mkdir -p "$MOUNT_POINT"
      - docker run -v "$MOUNT_POINT:/mnt" my-docker-image
    ```

An example project using this approach can be found here: https://gitlab.com/gitlab-examples/docker.

### Use Docker socket binding

The third approach is to bind-mount `/var/run/docker.sock` into the container so that docker is available in the context of that image.

In order to do that, follow the steps:

1. Install [GitLab Runner](https://docs.gitlab.com/runner/install).

1. Register GitLab Runner from the command line to use `docker` and share `/var/run/docker.sock`:

    ```bash
    sudo gitlab-runner register -n \
      --url https://gitlab.com/ \
      --registration-token REGISTRATION_TOKEN \
      --executor docker \
      --description "My Docker Runner" \
      --docker-image "docker:stable" \
      --docker-volumes /var/run/docker.sock:/var/run/docker.sock
    ```

    The above command will register a new Runner to use the special
    `docker:stable` image which is provided by Docker. **Notice that it's using
    the Docker daemon of the Runner itself, and any containers spawned by docker
    commands will be siblings of the Runner rather than children of the runner.**
    This may have complications and limitations that are unsuitable for your workflow.

    The above command will create a `config.toml` entry similar to this:

    ```
    [[runners]]
      url = "https://gitlab.com/"
      token = REGISTRATION_TOKEN
      executor = "docker"
      [runners.docker]
        tls_verify = false
        image = "docker:stable"
        privileged = false
        disable_cache = false
        volumes = ["/var/run/docker.sock:/var/run/docker.sock", "/cache"]
      [runners.cache]
        Insecure = false
    ```

1. You can now use `docker` in the build script (note that you don't need to
   include the `docker:dind` service as when using the Docker in Docker executor):

    ```yaml
    image: docker:stable

    before_script:
    - docker info

    build:
      stage: build
      script:
      - docker build -t my-docker-image .
      - docker run my-docker-image /script/to/run/tests
    ```

While the above method avoids using Docker in privileged mode, you should be
aware of the following implications:

- By sharing the docker daemon, you are effectively disabling all
  the security mechanisms of containers and exposing your host to privilege
  escalation which can lead to container breakout. For example, if a project
  ran `docker rm -f $(docker ps -a -q)` it would remove the GitLab Runner
  containers.
- Concurrent jobs may not work; if your tests
  create containers with specific names, they may conflict with each other.
- Sharing files and directories from the source repo into containers may not
  work as expected since volume mounting is done in the context of the host
  machine, not the build container, e.g.:

    ```
    docker run --rm -t -i -v $(pwd)/src:/home/app/src test-image:latest run_app_tests
    ```

## Making docker-in-docker builds faster with Docker layer caching

When using docker-in-docker, Docker will download all layers of your image every
time you create a build. Recent versions of Docker (Docker 1.13 and above) can
use a pre-existing image as a cache during the `docker build` step, considerably
speeding up the build process.

### How Docker caching works

When running `docker build`, each command in `Dockerfile` results in a layer.
These layers are kept around as a cache and can be reused if there haven't been
any changes. Change in one layer causes all subsequent layers to be recreated.

You can specify a tagged image to be used as a cache source for the `docker build`
command by using the `--cache-from` argument. Multiple images can be specified
as a cache source by using multiple `--cache-from` arguments. Keep in mind that
any image that's used with the `--cache-from` argument must first be pulled
(using `docker pull`) before it can be used as a cache source.

### Using Docker caching

Here's a simple `.gitlab-ci.yml` file showing how Docker caching can be utilized:

```yaml
image: docker:stable

services:
  - docker:dind

variables:
  CONTAINER_IMAGE: registry.gitlab.com/$CI_PROJECT_PATH
  DOCKER_HOST: tcp://docker:2375
  DOCKER_DRIVER: overlay2

before_script:
  - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN registry.gitlab.com

build:
  stage: build
  script:
    - docker pull $CONTAINER_IMAGE:latest || true
    - docker build --cache-from $CONTAINER_IMAGE:latest --tag $CONTAINER_IMAGE:$CI_BUILD_REF --tag $CONTAINER_IMAGE:latest .
    - docker push $CONTAINER_IMAGE:$CI_BUILD_REF
    - docker push $CONTAINER_IMAGE:latest
```

The steps in the `script` section for the `build` stage can be summed up to:

1. The first command tries to pull the image from the registry so that it can be
   used as a cache for the `docker build` command.
1. The second command builds a Docker image using the pulled image as a
   cache (notice the `--cache-from $CONTAINER_IMAGE:latest` argument) if
   available, and tags it.
1. The last two commands push the tagged Docker images to the container registry
   so that they may also be used as cache for subsequent builds.

## Using the OverlayFS driver

NOTE: **Note:**
The shared Runners on GitLab.com use the `overlay2` driver by default.

By default, when using `docker:dind`, Docker uses the `vfs` storage driver which
copies the filesystem on every run. This is a very disk-intensive operation
which can be avoided if a different driver is used, for example `overlay2`.

### Requirements

1. Make sure a recent kernel is used, preferably `>= 4.2`.
1. Check whether the `overlay` module is loaded:

    ```
    sudo lsmod | grep overlay
    ```

    If you see no result, then it isn't loaded. To load it use:

    ```
    sudo modprobe overlay
    ```

    If everything went fine, you need to make sure module is loaded on reboot.
    On Ubuntu systems, this is done by editing `/etc/modules`. Just add the
    following line into it:

    ```
    overlay
    ```

### Use driver per project

You can enable the driver for each project individually by editing the project's `.gitlab-ci.yml`:

```
variables:
  DOCKER_DRIVER: overlay2
```

### Use driver for every project

To enable the driver for every project, you can set the environment variable for every build by adding `environment` in the `[[runners]]` section of `config.toml`:

```toml
environment = ["DOCKER_DRIVER=overlay2"]
```

If you're running multiple Runners you will have to modify all configuration files.

> **Notes:**
- More information about the Runner configuration is available in the [Runner documentation](https://docs.gitlab.com/runner/configuration/).
- For more information about using OverlayFS with Docker, you can read
  [Use the OverlayFS storage driver](https://docs.docker.com/engine/userguide/storagedriver/overlayfs-driver/).

## Using the GitLab Container Registry

> **Notes:**
- This feature requires GitLab 8.8 and GitLab Runner 1.2.
- Starting from GitLab 8.12, if you have [2FA] enabled in your account, you need
  to pass a [personal access token][pat] instead of your password in order to
  login to GitLab's Container Registry.

Once you've built a Docker image, you can push it up to the built-in
[GitLab Container Registry](../../user/project/container_registry.md). For example,
if you're using docker-in-docker on your runners, this is how your `.gitlab-ci.yml`
could look like:

```yaml
 build:
   image: docker:stable
   services:
   - docker:dind
   variables:
     DOCKER_HOST: tcp://docker:2375
     DOCKER_DRIVER: overlay2
   stage: build
   script:
     - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN registry.example.com
     - docker build -t registry.example.com/group/project/image:latest .
     - docker push registry.example.com/group/project/image:latest
```

You have to use the special `gitlab-ci-token` user created for you in order to
push to the Registry connected to your project. Its password is provided in the
`$CI_JOB_TOKEN` variable. This allows you to automate building and deployment
of your Docker images.

You can also make use of [other variables](../variables/README.md) to avoid hardcoding:

```yaml
services:
  - docker:dind

variables:
  DOCKER_HOST: tcp://docker:2375
  DOCKER_DRIVER: overlay2
  IMAGE_TAG: $CI_REGISTRY_IMAGE:$CI_COMMIT_REF_SLUG

before_script:
  - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $CI_REGISTRY

build:
  stage: build
  script:
    - docker build -t $IMAGE_TAG .
    - docker push $IMAGE_TAG
```

Here, `$CI_REGISTRY_IMAGE` would be resolved to the address of the registry tied
to this project. Since `$CI_COMMIT_REF_NAME` resolves to the branch or tag name,
and your branch-name can contain forward slashes (e.g., feature/my-feature), it is
safer to use `$CI_COMMIT_REF_SLUG` as the image tag. This is due to that image tags
cannot contain forward slashes. We also declare our own variable, `$IMAGE_TAG`,
combining the two to save us some typing in the `script` section.

Here's a more elaborate example that splits up the tasks into 4 pipeline stages,
including two tests that run in parallel. The `build` is stored in the container
registry and used by subsequent stages, downloading the image
when needed. Changes to `master` also get tagged as `latest` and deployed using
an application-specific deploy script:

```yaml
image: docker:stable
services:
- docker:dind

stages:
- build
- test
- release
- deploy

variables:
  DOCKER_HOST: tcp://docker:2375
  DOCKER_DRIVER: overlay2
  CONTAINER_TEST_IMAGE: registry.example.com/my-group/my-project/my-image:$CI_COMMIT_REF_SLUG
  CONTAINER_RELEASE_IMAGE: registry.example.com/my-group/my-project/my-image:latest

before_script:
  - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN registry.example.com

build:
  stage: build
  script:
    - docker build --pull -t $CONTAINER_TEST_IMAGE .
    - docker push $CONTAINER_TEST_IMAGE

test1:
  stage: test
  script:
    - docker pull $CONTAINER_TEST_IMAGE
    - docker run $CONTAINER_TEST_IMAGE /script/to/run/tests

test2:
  stage: test
  script:
    - docker pull $CONTAINER_TEST_IMAGE
    - docker run $CONTAINER_TEST_IMAGE /script/to/run/another/test

release-image:
  stage: release
  script:
    - docker pull $CONTAINER_TEST_IMAGE
    - docker tag $CONTAINER_TEST_IMAGE $CONTAINER_RELEASE_IMAGE
    - docker push $CONTAINER_RELEASE_IMAGE
  only:
    - master

deploy:
  stage: deploy
  script:
    - ./deploy.sh
  only:
    - master
```

Some things you should be aware of when using the Container Registry:

- You must log in to the container registry before running commands. Putting
  this in `before_script` will run it before each job.
- Using `docker build --pull` makes sure that Docker fetches any changes to base
  images before building just in case your cache is stale. It takes slightly
  longer, but means you don’t get stuck without security patches to base images.
- Doing an explicit `docker pull` before each `docker run` makes sure to fetch
  the latest image that was just built. This is especially important if you are
  using multiple runners that cache images locally. Using the git SHA in your
  image tag makes this less necessary since each job will be unique and you
  shouldn't ever have a stale image, but it's still possible if you re-build a
  given commit after a dependency has changed.
- You don't want to build directly to `latest` in case there are multiple jobs
  happening simultaneously.

[docker-in-docker]: https://blog.docker.com/2013/09/docker-can-now-run-within-docker/
[docker-cap]: https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities
[2fa]: ../../user/profile/account/two_factor_authentication.md
[pat]: ../../user/profile/personal_access_tokens.md
