[ci] Use docker-coq-action, mathcomp/mathcomp and mathcomp/mathcomp-dev

[erikmd]

Subsumes #16

[erikmd]

I'm not sure to understand why, but it happens the last commit I pushed triggered a workflow in my fork only (for the push event), not in the math-comp/odd-order repo (for the pull_request event).

Even if the following thread is a bit old, it suggests this is related to a security mitigation:
https://github.community/t/run-a-github-action-on-pull-request-for-pr-opened-from-a-forked-repo/16054
but hopefully the CI status will directly work within the math-comp/odd-order PRs (including from forks) once the setup will be integrated in the default branch…

Anyway, let's wait for the on-going CI jobs to finish for the time being:
https://github.com/erikmd/odd-order/runs/730746813?check_suite_focus=true

[erikmd]

done, the CI is green - in my fork

[erikmd]

but maybe one should rename the opam file before merging this?

otherwise there is the:

Warning: the opam_file argument should have the '.opam' suffix.

[erikmd]

hence the following CI run: https://github.com/erikmd/odd-order/actions/runs/122529017