feat(gitlab-ci): enforce commit message format, build and release docker image

We want to tag images based on the different steps of the release cycle:

• user contribution branches have a nice ${CI_COMMIT_REF_SLUG} name for easier use
• dev images where developpement is integrated
• testing images where releases are stabilised
• stable images when the release is done
  • major tag with only the first digit of the semantic version, this tag will always point to the latest release of the major version
  • minor tag with only the first 2 digits of the semantic version, this tag will always point to the latest release of the minor version
  • release tag with the full semantic version
  • latest/stable point to the latest stable image

Closes #1 (closed)