After checking a couple of online examples, I created an example of git lab ci for moodle plugin ci testing

.gitlab-ci.yml

image: moodlehq/moodle-php-apache:7.3-stretch
services:
  - postgres:9.6

variables:
  MOODLE_BRANCH: "MOODLE_39_STABLE"
  DB: "pgsql"
  POSTGRES_DB: moodle
  POSTGRES_DB_USED: mdlunit
  POSTGRES_USER: moodle
  POSTGRES_PASSWORD: moodle
  TRAVIS_BUILD_DIR: "$CI_PROJECT_DIR"

# - Moodle installation MUST be outside $CI_PROJECT_DIR
# - moodle-plugin-ci MUST be in the $PATH
# - Database MUST be dropped prior to installation
# - Composer MUST be in the path for moodle-plugin-ci
# - Moodle 3.5 require Node v8 <9
before_script:
  - ls
  - apt-get update
  - seq 1 8 | xargs -I{} mkdir -p /usr/share/man/man{} # fix issue with postgresql-client on slim images. https://bit.ly/2JzEMZf
  - apt-get install -y git unzip postgresql-client-9.6
  - echo "Uninstall previous Moodle version"
  - export PGPASSWORD=$POSTGRES_PASSWORD
  - psql -h "postgres" -U "$POSTGRES_USER" -d "$POSTGRES_DB" -c "DROP DATABASE IF EXISTS $POSTGRES_DB_USED;" || true
  - echo "Install NPM"
  - curl -sL https://deb.nodesource.com/setup_14.x | bash -
  - apt-get install -y nodejs
  - cd ..
  - rm -rf -- ci
  # Install composer.
  - curl -sS https://getcomposer.org/installer | php
  - mv composer.phar /usr/local/bin/composer
  - composer create-project -n --no-dev --prefer-dist moodlehq/moodle-plugin-ci ci ^3
  - export PATH="$(cd ci/bin; pwd):$(cd ci/vendor/bin; pwd):$PATH"
  - chmod u+x ci/bin/moodle-plugin-ci
  - chmod u+x ci/bin/*
  - umask u+x
  - ls
  - pwd
  - moodle-plugin-ci install --db-user=$POSTGRES_USER --db-name=$POSTGRES_DB_USED --db-pass=$POSTGRES_PASSWORD --db-host=postgres -vvv
  - ls

.job_template: &job_definition
  script:
    # - moodle-plugin-ci codechecker
    - moodle-plugin-ci phpunit
    - moodle-plugin-ci phplint
    - moodle-plugin-ci phpcpd
    - moodle-plugin-ci phpmd
    - moodle-plugin-ci savepoints
    - moodle-plugin-ci mustache
    - moodle-plugin-ci grunt
    - moodle-plugin-ci validate
    # install docker inside of debian
    - apt-get install -y apt-transport-https ca-certificates gnupg lsb-release
    - curl -fsSL https://download.docker.com/linux/debian/gpg | gpg --dearmor -o /usr/share/keyrings/docker-archive-keyring.gpg
    - echo "deb [arch=amd64 signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/debian $(lsb_release -cs) stable" | tee /etc/apt/sources.list.d/docker.list > /dev/null
    - apt-get update
    - apt-get install -y docker-ce docker-ce-cli containerd.io
    - service docker start
    # run behat test using docker started selenium
    - moodle-plugin-ci behat
    - exit $EXIT_STATUS

Test:
  <<: *job_definition