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.