At Unleashed Technologies we use Gitlab CI with Docker runners for our continuous integration testing. We've put significant effort into speeding up the build execution speeds. One of the optimizations we made was to share a cache volume across all the CI jobs, allowing them to share files like package download caches.

Configuring the Docker runner was really simple - we simply dropped volumes = ["/srv/cache:/cache:rw"] into our config.toml file:

concurrent = 6 check_interval = 0 [[runners]] name = "ut-ci01" url = "https://gitlab.example.com/" token = "xxxxxxxxxxxxx" executor = "docker" [runners.docker] tls_verify = false image = "unleashed/php:7.1" privileged = false disable_cache = false volumes = ["/srv/cache:/cache:rw"] [runners.cache]

As a result, all CI jobs will have a /cache directory available (which is mapped to /srv/cache on the Docker host).

The next step was making the package managers utilize this cache directory whenever jobs run commands like composer install or yarn install . Luckilly these package managers allow us to configure their cache directories using environment variables:

Composer: COMPOSER_CACHE_DIR

Yarn: YARN_CACHE_FOLDER

npm: NPM_CONFIG_CACHE

bower: bower_storage__packages

RubyGems: GEM_SPEC_CACHE

pip: PIP_DOWNLOAD_CACHE

So we simply added these ENV directives in the Dockerfile s for our base images:

ENV COMPOSER_CACHE_DIR /cache/composer ENV YARN_CACHE_FOLDER /cache/yarn ENV NPM_CONFIG_CACHE /cache/npm ENV bower_storage__packages /cache/bower ENV GEM_SPEC_CACHE /cache/gem ENV PIP_DOWNLOAD_CACHE /cache/pip

Now, whenever a job needs a package installed, it'll pull from our local cache instead of downloading from a remote server! This provides a noticeable speed improvement for our builds.