GitLab CI with Docker - Step-by-Step Execution

Process Flow - GitLab CI with Docker

GitLab CI Pipeline Triggered

↓

GitLab Runner Starts Job

↓

Runner Uses Docker Executor

↓

Docker Container Created

↓

CI Script Runs Inside Container

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Job Completes

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Container Stops and Removes

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Pipeline Reports Status

This flow shows how GitLab CI triggers a job, runs it inside a Docker container, and reports the result.

Execution Sample

Docker

image: docker:latest

services:
  - docker:dind

script:
  - docker info
  - echo "Hello from Docker in GitLab CI"

This GitLab CI config runs a job using Docker image and Docker-in-Docker service, then prints Docker info and a message.

Process Table

Step	Action	Details	Result
1	Pipeline triggered	GitLab detects push or merge request	Pipeline starts
2	Runner picks job	GitLab Runner assigned to job	Runner starts job
3	Docker container created	Runner uses docker:latest image	Container ready
4	Docker-in-Docker service starts	docker:dind service runs alongside	Docker daemon available
5	Run 'docker info'	Inside container, command executed	Displays Docker daemon info
6	Run echo command	Print message inside container	Outputs: Hello from Docker in GitLab CI
7	Job completes	All script commands done	Job success
8	Container stops	Runner cleans up container	No leftover containers
9	Pipeline reports	GitLab shows job status	Success shown
10	Exit	No more steps	Pipeline finished

💡 Pipeline finishes after all script commands run and container is cleaned up

Status Tracker

Variable	Start	After Step 3	After Step 5	After Step 7	Final
Pipeline Status	Not started	Running	Running	Success	Success
Docker Container	None	Created	Running	Stopped	Removed
Docker Daemon	Not available	Starting	Available	Available	Stopped

Key Moments - 3 Insights

Why do we need the docker:dind service in GitLab CI?

The docker:dind service runs a Docker daemon inside the CI environment, allowing Docker commands like 'docker info' to work inside the container, as shown in execution_table step 4 and 5.

What happens to the Docker container after the job finishes?

After the job completes (step 7), the container stops and is removed by the runner to keep the environment clean, as seen in steps 8 and 9.

How does GitLab CI know the job succeeded?

GitLab CI checks the exit status of the script commands inside the container. If all commands succeed, the pipeline status changes to success (step 7 and 9).

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of the Docker container after step 5?

A Created but not running

B Stopped and removed

C Running inside the job

D Not created yet

💡 Hint

Check the 'Docker Container' variable in variable_tracker after Step 5

At which step does the GitLab Runner clean up the Docker container?

A Step 8

B Step 7

C Step 6

D Step 9

💡 Hint

Look at execution_table rows describing container stop and removal

If the docker:dind service was not included, what would happen at step 5?

A docker info would show daemon info as usual

B docker info would fail because no Docker daemon is running

C The job would skip docker info command

D The container would not start

💡 Hint

Refer to key_moments about why docker:dind service is needed

Concept Snapshot

GitLab CI with Docker:
- Use 'image: docker:latest' to run Docker commands
- Add 'services: - docker:dind' for Docker daemon
- Runner creates container, runs scripts inside
- Docker container stops and cleans after job
- Pipeline status reflects script success or failure

Full Transcript

This visual execution shows how GitLab CI uses Docker to run jobs. When a pipeline triggers, GitLab Runner starts a job using the docker:latest image. It also runs the docker:dind service to provide a Docker daemon inside the container. The job runs commands like 'docker info' and prints messages inside the container. After the job finishes, the container stops and is removed. GitLab reports the job status as success if all commands run without errors. This process ensures isolated, repeatable builds using Docker containers in GitLab CI.