ci(release): tag docker images based on release cycle

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