How to specify which docker image to use in .gitlab-ci.yml

The following .gitlab-ci.yml will build a native executable project using cmake with a custom docker image:

stages:
  - build

buildmyexe:
  stage: build
  image: 'ulikoehler/ubuntu-gcc-cmake:latest'
  script:
    - cmake .
    - make -j4

In this example, we have only one stage - if you have multiple stages, you can specify different images for each of them.