1. GitLab runs all CI/CD jobs within a Docker container. Ruby Docker image is used as default
   a. We can overwrite this default image by specifiying a image name globally (in the .gitlab-ci.yml file).

   image: <default_image>
   
   test_project:
       script:
           - pytest
   

   b. It is also allowed to use different docker images for different jobs. Each job will can have a key called image.

   image: <custom_image>
   
   # test python backend with pytest. So python is needed
   test_backend:
       image: python:3.9-slim-buster
       before_script:
           - pip install -r requirements.txt
       script:
           - pytest
   
   # test js frontend with npm. So node is needed
   test_frontend:
       image: node:alpine
       before_script:
           - npm install
       script:
           - npm test
   
   # test the integration. Some custom environment is needed
   test_integration:
       script:
           - make test # run within contianer of <custom_image> container
   

2. We can import jobs from other files by the URL of GitLab file like below
   lint.yml (https://gitlab.com/<org-name>/<repo-name>/-/blob/master/lint.yml):

   lint_python:
       stage: lint
       image: <image_name>
       services:
           - postgres:12.4-alpine
       variables:
           POSTGRES_DB: <db_name>
           POSTGRES_USER: <user_name>
           POSTGRES_PASSWORD: <password>
       before_script:
           - pip install --upgrade pip
           - pip3 install tox flake8
       script:
           - flake8 --statistics
   

   .gitlab-ci.yml:

   # refer to the above lint.yml
   include:
   - project: '<org-name>/<repo-name>/'
       ref: master
       file: 'lint.yml'
   
   # Specify stages
   stages:
       - lint
   

3. We can override the imported Jobs like below:

   .gitlab-ci.yml:

   # refer to the above lint.yml
   include:
   - project: '<org-name>/<repo-name>/ci-cd'
       ref: master
       file: '/templates/jobs/lint.yml'
   
   # Specify stages
   stages:
       - lint
   
   # Overwrite imported lint_python job from lint.yml
   lint_python:
       allow_failure: true
       script:
           - pip3 install black
           - black .
   

4. Since all Jobs run inside containers, when we need to execute docker commands (within a build Job for example), we will need docker to be available inside the Job's container. So we have to use docker in docker.

   a. A Service container is an additioanl container that starts at the same time as the Job container. Most common usecase is to run a database container.
   b. The services attribute will make sure that the Service container and the Job container will be on the same network and can talk to each other.

   build_image:
       stage: build
       image: docker:20.10.16 # for making the docker client available within Job's docker container
       services:
           - docker:20.10.16-dind # for making the docker daemon available within Job's docker container
           entrypoint: ["dockerd-entrypoint.sh", "--tls=false"] # 
       before_script:
           - docker login -u $REGISTRY_USER -p $REGISTRY_PASS
       script:
           - docker build -t $IMAGE_NAME:$IMAGE_TAG .
           - docker push $IMAGE_NAME:$IMAGE_TAG
   

   PS: With dind as service, we need to configure TLS, else we can disable it like above. Reference

5. Add a (.) in front of a job name and it will be skipped from execution. It is like commenting.

   .test_experiment: # skipped from execution
       script:
           - make test
   
   test_production:
       script:
           - pytest
   

6. Apart from secret variables, we can add custom variables in the .gitlab-ci.yml file to avoid repitition. They can either be defined per Job or globally. Just like secret variables, they are also referred with the help of $variable.

References:

1. GitLab CI/CD course by Tech with Nana