How

• Enable Builds in your GitLab project by going to the Edit Project page
• Setup a GitLab CI Runner using docker image php (I use https://githost.io for this, which is ran by GitLab)
• Add the files below to the root of your repo
• composer install
• Change the included directories in phpcs.xml
• Add/commit all new files
• Push, and watch the magic happen in the new Pipelines area of your GitLab project

Files

.gitlab-ci.yml

before_script:
  - apt-get update && apt-get install -y git
  - php bin/composer.phar install --dev --prefer-dist

stages:
  - syntax

phpcs:
  stage: syntax
  script:
    - ./vendor/bin/phpcs --error-severity=1 --warning-severity=8 --extensions=php

phpcs.xml

<?xml version="1.0"?>
<ruleset name="ASH2">
<description>The ASH2 coding standard.</description>
	<rule ref="PSR2"/>

	<file>codebase/src/</file>
	<file>codebase/tests/</file>
	<file>codebase/api/</file>

	<exclude-pattern>vendor</exclude-pattern>
</ruleset>

composer.json

{
    "require": {
        "squizlabs/php_codesniffer": "~2.7"
    }
}

Final thoughts

This would be much faster if we didn't have to install git for composer usage - either by having composer not use git, or by using a different docker image.