DO-58: cache xain-fl dependencies in Docker

[little-dude]

References

https://xainag.atlassian.net/browse/DO-58

Summary

Some dependencies require a very long compilation: protobuf, numpy,
grpcio. To avoid having to re-install these packages every time, we
pre-install them so that they are cached.

However, we cannot just pre-install a few packages, because we don't
know exactly which version pip will pick for them. Instead, we
give pip all the package's dependencies, and let it resolve and
install them.

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Reviewer checklist

Reviewer agreement:

• Reviewers assign themselves at the start of the review.
• Reviewers do not commit or merge the merge request.
• Reviewers have to check and mark items in the checklist.

Merge request checklist

• Conforms to the merge request title naming XP-XXX <a description in imperative form>.
• Each commit conforms to the naming convention XP-XXX <a description in imperative form>.
• Linked the ticket in the merge request title or the references section.
• Added an informative merge request summary.

Code checklist

• Conforms to the branch naming XP-XXX-<a_small_stub>.
• Passed scope checks.
• Added or updated tests if needed.
• Added or updated code documentation if needed.
• Conforms to Google docstring style.
• Conforms to XAIN structlog style.