Speed up Gitlab CI reusing docker machine for stages

Question

Gitlab CI pulls docker image every time for every task (stage). This operation wastes much time. I want to optimize if possible.

I see two places to work with:
1. explicitly configure CI stages to reuse the same docker machine.
2. use the docker machine from previous commit when building next commit? (If no changes in configuration file was done).

Answer 1

This kind of configuration can be specified trough the pull_policy on the runner itself.

As Jakub highlighted in the comments to the question, on shared runners on Gitlab.com the policy is set to always, therefore it will always download a new copy of the image, also if there is the same copy locally.

This due security reasons.

You can have a confirmation of that in the doc.

This pull policy should be used if your Runner is publicly available and configured as a shared Runner in your GitLab instance. It is the only pull policy that can be considered as secure when the Runner will be used with private images.

The security implication is that if the runner checks first a local image, a non authorized user can get a private docker image guessing its name