Allow running CI with changed external dependencies

[rlepigre-skylabs-ai]

Currently, due to how we manage docker images, it is not possible to easily try a new version of OCaml dependencies (or OCaml itself) without doing manual steps (mainly, building and deploying a new docker image).

It would be great if we could set up CI so that, if the dependencies change (currently, they are tracked in fmdeps/fm-ci/br-fm-deps.opam, but should probably move to the workspace), a docker image is re-build as part of stage 0, and then used in the following stages.

This would be extremely useful to, for example, try out OCaml 5.4. It would be even better if performance measurements could still be performed, by running the base version on the old image, and only the new job in the new image. I believe that aggregating the results could entirely be done in the new image, since it's mostly text files after data collection.

[pgiarrusso-sl]

It would be even better if performance measurements could still be performed, by running the base version on the old image, and only the new job in the new image.

For that, we should probably split measurements in 2 jobs and pass artifacts — that might also reduce latency.

[rlepigre-skylabs-ai]

Yes, I also believe that this would be necessary.

It might be good enough, as we can make such artifacts expire very quickly, so as to not exhaust our storage space.

[pgiarrusso-sl]

a docker image is re-build as part of stage 0, and then used in the following stages.

That might be too much — all we need here is supporting different image tags.

Reliable auto-builds raise questions about caching, artifacts, etc — they're good but quite a bit harder.