README.md

GitLab CI template for Docker

This project implements a GitLab CI/CD template to build, check and inspect your containers with Docker.

Usage

This template can be used both as a CI/CD component or using the legacy include:project syntax.

Use as a CI/CD component

Add the following to your .gitlab-ci.yml:

include:
  # 1: include the component
  - component: $CI_SERVER_FQDN/to-be-continuous/docker/gitlab-ci-docker@5.11.0
    # 2: set/override component inputs
    inputs:
      build-tool: buildah # ⚠ this is only an example

Use as a CI/CD template (legacy)

Add the following to your .gitlab-ci.yml:

include:
  # 1: include the template
  - project: 'to-be-continuous/docker'
    ref: '5.11.0'
    file: '/templates/gitlab-ci-docker.yml'

variables:
  # 2: set/override template variables
  DOCKER_BUILD_TOOL: buildah # ⚠ this is only an example

Understanding the Docker template

The template supports following ways of building container images:

1. The former Docker-in-Docker (DinD) technique, that was widely used for years because of no other alternative, but that is now commonly recognized to have significant security issues (read this post for more info),
2. Or using kaniko, an open-source, daemonless tool from Google for building Docker images, and that solves Docker-in-Docker security issues (and also speeds-up build times).
3. Or using buildah, an open-source, daemonless tool backed by RedHat for building Docker images, and that solves Docker-in-Docker security issues (and also speeds-up build times), and can also be configured to run rootless.

By default, the template uses the kaniko way, but you may select an alternate build tool by using the DOCKER_BUILD_TOOL variable (see below).

⚠️ If you choose to use 'Docker-in-Docker' option considering the associated security risks, make sure your runner has required privileges to run Docker-in-Docker (see GitLab doc).

Global variables

The Docker template uses some global configuration used throughout all jobs.

Input / Variable	Description	Default value
build-tool / DOCKER_BUILD_TOOL	The build tool to use for building container image, possible values are kaniko, buildah or dind	kaniko
kaniko-image / DOCKER_KANIKO_IMAGE	The image used to run kaniko - for kaniko build only	gcr.io/kaniko-project/executor:debug (use debug images for GitLab)
buildah-image / DOCKER_BUILDAH_IMAGE	The image used to run buildah - for buildah build only	quay.io/buildah/stable
image / DOCKER_IMAGE	The Docker image used to run the docker client (see full list) - for Docker-in-Docker build only	registry.hub.docker.com/library/docker:latest
dind-image / DOCKER_DIND_IMAGE	The Docker image used to run the Docker daemon (see full list) - for Docker-in-Docker build only	registry.hub.docker.com/library/docker:dind
file / DOCKER_FILE	The path to your Dockerfile	Dockerfile
context-path / DOCKER_CONTEXT_PATH	The Docker context path (working directory)	none only set if you want a context path different from the Dockerfile location

In addition to this, the template supports standard Linux proxy variables:

Input / Variable	Description	Default value
http_proxy	Proxy used for http requests	none
https_proxy	Proxy used for https requests	none
no_proxy	List of comma-separated hosts/host suffixes	none

Images

For each Dockerfile, the template builds an image that may be pushed as two distinct images, depending on a certain workflow:

1. snapshot: the image is first built from the Dockerfile and then pushed to some Docker registry as the snapshot image. It can be seen as the raw result of the build, but still untested and unreliable.
2. release: once the snapshot image has been thoroughly tested (both by package-test stage jobs and/or acceptance stage jobs after being deployed to some server), then the image is pushed one more time as the release image. This second push can be seen as the promotion of the snapshot image being now tested and reliable.

In practice:

• the snapshot image is always pushed by the template (pipeline triggered by a Git tag or commit on any branch),
• the release image is only pushed:
  • on a pipeline triggered by a Git tag,
  • on a pipeline triggered by a Git commit on master.

The snapshot and release images are defined by the following variables:

Input / Variable	Description	Default value
snapshot-image / DOCKER_SNAPSHOT_IMAGE	Docker snapshot image	$CI_REGISTRY_IMAGE/snapshot:$CI_COMMIT_REF_SLUG
release-image / DOCKER_RELEASE_IMAGE	Docker release image	$CI_REGISTRY_IMAGE:$CI_COMMIT_REF_NAME

As you can see, the Docker template is configured by default to use the GitLab container registry. You may perfectly override this and use another Docker registry, but be aware of a few things:

• the DOCKER_SNAPSHOT_IMAGE requires a Docker registry that allows tag overwrite,
• the DOCKER_RELEASE_IMAGE may use a Docker registry that doesn't allow tag overwrite, but:
  1. you should avoid overwriting a Git tag (at it will obviously fail while trying to (re)push the Docker image),
  2. you have to deactivate publish on main (or master) branch by setting the $DOCKER_PROD_PUBLISH_STRATEGY variable to none (as it would lead to the main tag being overwritten).

Registries and credentials

As seen in the previous chapter, the Docker template uses by default the GitLab registry to push snapshot and release images. Thus it makes use of credentials provided by GitLab itself to login (CI_REGISTRY_USER / CI_REGISTRY_PASSWORD).

But when using other registry(ies), you'll have also to configure appropriate Docker credentials.

Using the same registry for snapshot and release

If you use the same registry for both snapshot and release images, you shall use the following configuration

variables:

Input / Variable	Description
🔒 DOCKER_REGISTRY_USER	Docker registry username for image registry
🔒 DOCKER_REGISTRY_PASSWORD	Docker registry password for image registry

Using different registries for snapshot and release

If you use different registries for snapshot and release images, you shall use separate configuration variables:

Input / Variable	Description
🔒 DOCKER_REGISTRY_SNAPSHOT_USER	Docker registry username for snapshot image registry
🔒 DOCKER_REGISTRY_SNAPSHOT_PASSWORD	Docker registry password for snapshot image registry
🔒 DOCKER_REGISTRY_RELEASE_USER	Docker registry username for release image registry
🔒 DOCKER_REGISTRY_RELEASE_PASSWORD	Docker registry password for release image registry

Setting your own Docker configuration file (advanced)

There might be cases where you need to provide the complete Docker configuration file:

• need to declare authentication credentials for other registries than the 2 predefined ones (snapshot & release),
• need to declare a credentials store (ex: in order to publish to Amazon ECR with Kaniko for instance),
• need to declare proxies,
• ...

If you are in one of those cases, you will need to use the DOCKER_CONFIG_FILE variable, expected to declare the path to your custom Docker configuration file (JSON). You may:

• leave the default value (.docker/config.json) or override it to some alternate location in your project repository and create the file without any secret in it using our dynamic variables replacement (see below),
• or override it as a GitLab project variable of type File, possibly inlining your secret credentials in it.

Input / Variable	Description	Default value
config-file / DOCKER_CONFIG_FILE	Path to the Docker configuration file (JSON)	.docker/config.json

Moreover, this file supports dynamic environment variables replacement. That means it may contain references to other environment variables (in the format ${variable_name}) that will be dynamically replaced by the template before evaluation. In addition to you own defined variables, you may use the following variables (provided and managed by the template):

• ${docker_snapshot_authent_token}: the authentication token required by the snapshot registry (computed from configured DOCKER_REGISTRY_SNAPSHOT_USER / DOCKER_REGISTRY_SNAPSHOT_PASSWORD variables)
• ${docker_snapshot_registry_host}: the snapshot registry host (based on the configured DOCKER_SNAPSHOT_IMAGE variable)
• ${docker_release_authent_token}: the authentication token required by the release registry (computed from configured DOCKER_REGISTRY_RELEASE_USER / DOCKER_REGISTRY_RELEASE_PASSWORD variables)
• ${docker_release_registry_host}: the release registry host (based on the configured DOCKER_RELEASE_IMAGE variable)

Example 1: Docker configuration file inlined in the project repository (.docker/config.json) with dynamic variables replacement:

{
    "auths": {
        "${docker_snapshot_registry_host}": {
            "auth": "${docker_release_authent_token}"
        },
        "${docker_release_registry_host}": {
            "auth": "${docker_snapshot_authent_token}"
        },
        "my-readonly-repo-to-pull": {
            "auth": "${MY_OWN_REGISTRY_TOKEN}"
        }
    }
}

This file uses:

• template-managed ${docker_snapshot_authent_token}, ${docker_snapshot_registry_host}, ${docker_release_authent_token} and ${docker_release_registry_host} variables,
• the user-defined ${MY_OWN_REGISTRY_TOKEN} (ℹ️ an authentication token can be obtained with command echo "user:password" | base64 and then be stored as a masked GitLab CI/CD project variable).

Example 2: Docker configuration file declared as a GitLab project variable of type File with dynamic variables replacement:

{
    "auths": {
        "$${docker_snapshot_registry_host}": {
            "auth": "$${docker_release_authent_token}"
        },
        "$${docker_release_registry_host}": {
            "auth": "$${docker_snapshot_authent_token}"
        },
        "my-readonly-repo-to-pull": {
            "auth": "ZG9ja2VyZHVkZTpnb3RjaGEh"
        }
    }
}

This file uses:

• template-managed ${docker_snapshot_authent_token}, ${docker_snapshot_registry_host}, ${docker_release_authent_token} and ${docker_release_registry_host} variables (⚠️ mind the double $$ to prevent GitLab from trying to evaluate the variable),
• the user-defined authentication may be inlined as a GitLab project variable is a place safe enough to store secrets.

Multi Dockerfile support

This template supports building multiple Docker images from a single Git repository.

You can define the images to build using the parallel matrix jobs pattern inside the .docker-base job (this is the top parent job of all Docker template jobs).

Since each job in the template extends this base job, the pipeline will produce one job instance per image to build. You can independently configure each instance of these jobs by redefining the variables described throughout this documentation.

For example, if you want to build two Docker images, you must specify where the Dockerfiles are located and where the resulting images will be stored. You can do so by adding a patch to the .docker-base job in your .gitlab-ci.yml file so that it looks like this:

.docker-base:
  parallel:
    matrix:
    - DOCKER_FILE: "front/Dockerfile"
      DOCKER_SNAPSHOT_IMAGE: "$CI_REGISTRY_IMAGE/front:$CI_COMMIT_REF_SLUG"
      DOCKER_RELEASE_IMAGE: "$CI_REGISTRY_IMAGE/front:$CI_COMMIT_REF_NAME"
    - DOCKER_FILE: "back/Dockerfile"
      DOCKER_SNAPSHOT_IMAGE: "$CI_REGISTRY_IMAGE/back:$CI_COMMIT_REF_SLUG"
      DOCKER_RELEASE_IMAGE: "$CI_REGISTRY_IMAGE/back:$CI_COMMIT_REF_NAME"

If you need to redefine a variable with the same value for all your Dockerfiles, you can just declare this variable as a global variable. For example, if you want to build all your images using buildah, you can simply define the DOCKER_BUILD_TOOL variable as a global variable with value buildah:

variables:
  DOCKER_BUILD_TOOL: "buildah"

Secrets management

Here are some advices about your secrets (variables marked with a 🔒):

1. Manage them as project or group CI/CD variables:
   • masked to prevent them from being inadvertently displayed in your job logs,
   • protected if you want to secure some secrets you don't want everyone in the project to have access to (for instance production secrets).
2. In case a secret contains characters that prevent it from being masked, simply define its value as the Base64 encoded value prefixed with @b64@: it will then be possible to mask it and the template will automatically decode it prior to using it.
3. Don't forget to escape special characters (ex: $ -> $$).

Jobs

docker-hadolint job

This job performs a Lint on your Dockerfile.

It is bound to the build stage, and uses the following variables:

Input / Variable	Description	Default value
hadolint-disabled / DOCKER_HADOLINT_DISABLED	Set to true to disable Hadolint	(none: enabled by default)
hadolint-image / DOCKER_HADOLINT_IMAGE	The Hadolint image	registry.hub.docker.com/hadolint/hadolint:latest-alpine
hadolint-args / DOCKER_HADOLINT_ARGS	Additional hadolint arguments	(none)

In case you have to disable some rules, either add --ignore XXXX to the DOCKER_HADOLINT_ARGS variable or create a Hadolint configuration file named hadolint.yaml at the root of your repository.

You can also use inline ignores in your Dockerfile:

# hadolint ignore=DL3006
FROM ubuntu

# hadolint ignore=DL3003,SC1035
RUN cd /tmp && echo "hello!"

In addition to a textual report in the console, this job produces the following reports, kept for one day:

Report	Format	Usage
reports/docker-hadolint-*.native.json	native hadolint test report (json)	DefectDojo integration This report is generated only if DefectDojo template is detected
reports/docker-hadolint-*.codeclimate.json	hadolint (GitLab) codeclimate format	GitLab integration

docker-*-build jobs

This job builds the image and publishes it to the snapshot repository.

It is bound to the package-build stage, and uses the following variables:

Input / Variable	Description	Default value
build-args / DOCKER_BUILD_ARGS	Additional docker/kaniko/buildah build arguments	(none)
registry-mirror / DOCKER_REGISTRY_MIRROR	URL of a Docker registry mirror to use during the image build (instead of default https://index.docker.io) ⚠️ Used by the kaniko and dind options only	(none)
container-registries-config-file / CONTAINER_REGISTRIES_CONFIG_FILE	The registries.conf configuration to be used ⚠️ Used by the buildah build only	(none)
metadata / DOCKER_METADATA	Additional docker build/kaniko arguments to set label	OCI Image Format Specification
kaniko-snapshot-image-cache / KANIKO_SNAPSHOT_IMAGE_CACHE	Snapshot image repository that will be used to store cached layers (leave empty to use default: snapshot image repository + /cache) ⚠️ Used by the kaniko build only	none (default cache path)
build-cache-disabled / DOCKER_BUILD_CACHE_DISABLED	Set to true to disable the build cache. Cache can typically be disabled when there is a network latency between the container registry and the runner.	none (i.e cache enabled)

This job produces output variables that are propagated to downstream jobs (using dotenv artifacts):

Input / Variable	Description	Example
docker_image	snapshot image name with tag	registry.gitlab.com/acme/website/snapshot:main
docker_image_digest	snapshot image name with digest (no tag)	registry.gitlab.com/acme/website/snapshot@sha256:b7914a91...
docker_repository	snapshot image bare repository (no tag nor digest)	registry.gitlab.com/acme/website/snapshot
docker_tag	snapshot image tag	main
docker_digest	snapshot image digest	sha256:b7914a91...

They may be freely used in downstream jobs (for instance to deploy the upstream built Docker image, whatever the branch or tag).

If you want to use GitLab CI variables or any other variable in your Dockerfile, you can add them to DOCKER_BUILD_ARGS like so:

DOCKER_BUILD_ARGS: "--build-arg CI_PROJECT_URL --build-arg MY_VAR='MY_VALUE'"

These variables will then be available for use in your Dockerfile:

FROM scratch

ARG CI_PROJECT_URL
ARG MY_VAR
LABEL name="my-project"                   \
      description="My Project: $MY_VAR"   \
      url=$CI_PROJECT_URL                 \
      maintainer="my-project@acme.com"

Default value for DOCKER_METADATA supports a subset of the OCI Image Format Specification for labels and use GitLab CI pre-defined variables to guess the value as follow :

Label	GitLab CI pre-defined variable
org.opencontainers.image.url	$CI_PROJECT_URL
org.opencontainers.image.source	$CI_PROJECT_URL
org.opencontainers.image.title	$CI_PROJECT_PATH
org.opencontainers.image.ref.name	$CI_COMMIT_REF_NAME
org.opencontainers.image.revision	$CI_COMMIT_SHA
org.opencontainers.image.created	$CI_JOB_STARTED_AT

Note that spaces are currently not supported by Kaniko. Therefore, title couldn't be CI_PROJECT_TITLE.