GitLab CI with Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

GitLab CI Runner Docker Image Output

What is the output of the following command when run inside a GitLab CI job using the official Docker image docker:latest?

Docker

docker --version

A Docker version 20.10.23, build 7155243

B Error: Cannot connect to the Docker daemon

C Docker version 18.09.1, build 4c52b90

D bash: docker: command not found

💡 Hint

The official Docker image used in GitLab CI jobs includes the Docker CLI by default.

The docker:latest image includes the Docker CLI, so running docker --version inside a job will result in the Docker version being printed.

❓ Configuration

intermediate

Correct Docker-in-Docker Service Configuration

Which GitLab CI configuration snippet correctly enables Docker-in-Docker (DinD) service for building Docker images inside a job?

A

services:
  - docker:dind
variables:
  DOCKER_HOST: tcp://docker:2375
  DOCKER_TLS_CERTDIR: ""

B

services:
  - docker:latest
variables:
  DOCKER_HOST: unix:///var/run/docker.sock

C

services:
  - docker:dind
variables:
  DOCKER_HOST: unix:///var/run/docker.sock

D

services:
  - docker:dind
variables:
  DOCKER_HOST: tcp://localhost:2375
  DOCKER_TLS_CERTDIR: ""

💡 Hint

DinD requires connecting to the Docker daemon over TCP and disabling TLS cert directory.

The correct setup uses the docker:dind service with DOCKER_HOST set to tcp://docker:2375 and disables TLS by setting DOCKER_TLS_CERTDIR to an empty string.

🔀 Workflow

advanced

Order of Steps to Build and Push Docker Image in GitLab CI

Arrange the following steps in the correct order to build and push a Docker image using GitLab CI with Docker-in-Docker service.

A 1,2,4,3

B 1,4,2,3

C 4,2,1,3

D 4,1,2,3

💡 Hint

You must prepare the environment before logging in and building images.

First, set up the DinD service and variables (4), then login to the registry (1), build the image (2), and finally push it (3).

❓ Troubleshoot

advanced

Diagnosing Docker Push Failure in GitLab CI

A GitLab CI job using Docker-in-Docker fails with the error: denied: requested access to the resource is denied when pushing an image. What is the most likely cause?

A Docker image build failed before push

B Docker-in-Docker service is not started

C Docker registry login credentials are missing or incorrect

D GitLab runner does not have network access

💡 Hint

Access denied errors usually relate to authentication.

The error indicates the push was rejected due to lack of permission, commonly caused by missing or wrong login credentials.

✅ Best Practice

expert

Best Practice for Secure Docker-in-Docker Usage in GitLab CI

Which practice is recommended to securely use Docker-in-Docker in GitLab CI pipelines?

A Disable TLS and use plain TCP connection for simplicity

B Run DinD service with TLS enabled and use certificates for authentication

C Use Docker socket binding from host to avoid DinD service

D Run Docker commands directly on the GitLab runner host

💡 Hint

Security is important when exposing Docker daemon.

Enabling TLS and using certificates protects the Docker daemon communication, which is best practice for DinD security.

Practice

1. What is the main purpose of using docker:dind service in a GitLab CI pipeline?

easy

A. To send notifications after the pipeline finishes

B. To deploy the application to production automatically

C. To run Docker commands inside the CI job container

D. To store build artifacts permanently

Solution

1. Step 1: Understand the role of docker:dind

   The docker:dind (Docker-in-Docker) service allows Docker commands to run inside a container, which is essential for building and managing Docker images during CI jobs.

2. Step 2: Identify the purpose in GitLab CI context

   In GitLab CI, using docker:dind lets the pipeline build, run, and push Docker images isolated from the host system.

3. Final Answer:

   To run Docker commands inside the CI job container -> Option C

4. Quick Check:

   docker:dind = run Docker commands in CI [OK]

Hint: Remember: dind means Docker runs inside Docker [OK]

Common Mistakes:

• Confusing dind with deployment step
• Thinking dind stores artifacts
• Assuming dind sends notifications

2. Which of the following is the correct way to specify the Docker service in a GitLab CI job to enable Docker commands?

easy

A. services:
- docker:dind

B. services:
- docker:latest

C. services:
- docker:build

D. services:
- docker:run

Solution

1. Step 1: Recall the correct service name for Docker-in-Docker

   The official Docker-in-Docker service image is docker:dind, which must be specified exactly to enable Docker commands in GitLab CI.

2. Step 2: Compare options to find exact match

   Only services: - docker:dind uses the correct service name docker:dind. Other options are invalid or do not exist as official images.

3. Final Answer:

   services: - docker:dind -> Option A

4. Quick Check:

   Correct service name = docker:dind [OK]

Hint: Use exact service name 'docker:dind' for Docker commands [OK]

Common Mistakes:

• Using 'docker:latest' instead of 'docker:dind'
• Adding extra tags like 'build' or 'run'
• Omitting the services section

3. Given this GitLab CI job snippet, what will be the output of the docker ps command inside the job?

job:
  image: docker:latest
  services:
    - docker:dind
  script:
    - docker info
    - docker ps

medium

A. Shows an error: Docker daemon not running

B. Lists running containers inside the dind service (likely empty)

C. Lists containers on the host machine

D. Outputs the Docker version only

Solution

1. Step 1: Understand the environment of the job

   The job uses docker:dind service, which runs a Docker daemon inside the CI environment. The docker ps command connects to this daemon.

2. Step 2: Predict the output of docker ps

   Since the job just started and no containers were run inside the dind daemon, docker ps will list running containers inside the dind environment, which is likely empty.

3. Final Answer:

   Lists running containers inside the dind service (likely empty) -> Option B

4. Quick Check:

   docker ps inside dind = running containers (empty if none) [OK]

Hint: docker ps shows containers inside dind, not host [OK]

Common Mistakes:

• Expecting error about Docker daemon
• Thinking it lists host machine containers
• Confusing docker info output with docker ps

4. You have this GitLab CI job configuration:

build:
  image: docker:latest
  script:
    - docker build -t myapp .
  services:
    - docker:dind

The job fails with error: Cannot connect to the Docker daemon. What is the most likely fix?

medium

A. Add variables:
DOCKER_HOST: tcp://docker:2375 to the job

B. Change image to docker:dind

C. Remove the services section

D. Add docker login before build

Solution

1. Step 1: Identify cause of Docker daemon connection error

   The error means the Docker client inside the job cannot reach the Docker daemon running in the docker:dind service container.

2. Step 2: Fix connection by setting DOCKER_HOST variable

   Setting DOCKER_HOST: tcp://docker:2375 tells the Docker client to connect to the dind service over TCP, enabling communication.

3. Final Answer:

   Add variables: DOCKER_HOST: tcp://docker:2375 to the job -> Option A

4. Quick Check:

   Set DOCKER_HOST to connect client to dind daemon [OK]

Hint: Set DOCKER_HOST to tcp://docker:2375 to fix connection [OK]

Common Mistakes:

• Changing image to docker:dind instead of using service
• Removing services section disables Docker daemon
• Adding docker login does not fix daemon connection

5. You want to build a Docker image and push it to GitLab Container Registry in a GitLab CI job. Which snippet correctly sets up the job with Docker-in-Docker and pushes the image?

build_and_push:
  image: docker:latest
  services:
    - docker:dind
  variables:
    DOCKER_HOST: tcp://docker:2375
    DOCKER_TLS_CERTDIR: ""
  script:
    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
    - docker build -t $CI_REGISTRY_IMAGE:latest .
    - docker push $CI_REGISTRY_IMAGE:latest

hard

A. No docker login command before push

B. Missing DOCKER_HOST variable, so Docker commands fail

C. Uses docker:dind as image instead of service

D. Correct setup with variables and login before build and push

Solution

1. Step 1: Check Docker-in-Docker setup

   The job uses docker:dind as a service and sets DOCKER_HOST and disables TLS cert dir, which is required for Docker commands to connect properly.

2. Step 2: Verify login and push steps

   The job logs into the GitLab Container Registry using CI variables, then builds and pushes the image with correct tags.

3. Final Answer:

   Correct setup with variables and login before build and push -> Option D

4. Quick Check:

   Proper dind setup + login + build + push = Correct setup with variables and login before build and push [OK]

Hint: Always login before push and set DOCKER_HOST for dind [OK]

Common Mistakes:

• Forgetting DOCKER_HOST causes connection errors
• Using docker:dind as image instead of service
• Skipping docker login before push