Add Docker-based GitLab CI configuration

* Design:
  - build stage (e.g. docker build -t mathcomp-dev:$IID_$SLUG_coq-8.6 .)
    - choice of the OCaml compiler: var OPAM_SWITCH in {base, edge}
      (Dockerfile containing: "opam switch set $compiler && eval $(opam env)")
    - master (protected branch) => push on GitLab registry and Docker Hub
    - other branches (not tags) => push on GitLab registry
    - Todo: GitHub PRs => push on GitLab
  - test stage (image: mathcomp-dev:$IID_$SLUG_coq-8.6)
    - script template foreach project (custom CONTRIB_URL, script)
    - jobs foreach project and Coq version (custom COQ_VERSION, CONTRIB_VERSION)

* Config for protected branches:
  - set vars HUB_REGISTRY, HUB_REGISTRY_USER, HUB_REGISTRY_IMAGE, HUB_TOKEN

* Remark:
  - The name chosen for branches should ideally yield different values
    of CI_COMMIT_REF_SLUG.
  - But this is not mandatory as image tags start with "${CI_PIPELINE_IID}_".
  cf. doc:
  - CI_COMMIT_REF_NAME: The branch or tag name for which project is built.
  - CI_COMMIT_REF_SLUG: $CI_COMMIT_REF_NAME lowercased, shortened to 63 bytes,
    and with everything except 0-9 and a-z replaced with -.
    No leading / trailing -. Use in URLs, host names and domain names.
  - CI_PIPELINE_IID: The unique id of the current pipeline scoped to project.

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